ATH-M50 prices have gone up (way up) and so the next logical choice was the ATH-M40, which was half the price. ATH-M40 has 40 mm. diaphragms unlike ATH-M50's 50 mm. Otherwise it's a fairly similar model; the accuracy is there too, although they're slightly colder (and faster). ATH-M40 have straight drivers, unlike ATH-M50, which have angled drivers. IMHO that is the only flaw with the ATH-M50: angled drivers bleed some soundstage and midrange power. Though the ATH-M50 drivers are engineered to compensate, they still have a few slight holes here and there in the frequency response. Angled drivers are a sales gimmick really, though A-T engineers did a good job with the ATH-M50. ATH-M40's straight drivers are therefore an advantage over the ATH-M50.
 
ATH-M40 is Audio-Technica's "straight frequency response" monitor model. Now in fairness, "straight response" for headphones is one that's adapted to your own HRTF frequency response. These are equalised to an average HRTF, but yes, they are quite "straight" in response. With one caveat...
 
ATH-M40 take a while to burn-in, and the burn-in transformation is the most obvious case. Out of the box, they were snowy-white in sound. Not even black-and-white, grays-on-white. No contrast or colour whatsoever. It even looked like AT made a blunder and equalised those to a "speaker-flat" frequency response. A special EQ profile had to be made to make their sound bearable (adjusted to HRTF with EQ band tones), and apparently this EQ profile helped, as after a week or so of playback they started warming up, and on the second week the EQ preset could be discarded.
 
Here's the original Foobar2000 EQ:
 

 
Now they play fine without it.
 
The design itself is curious, ATH-M40 boast clamp wire terminals. Unlike most headphones, the ATH-M40 are designed for cable replacement without soldering. Main cable terminals are of the screw clamp type, similar to speakers. Wires are inserted and then screws push down on the terminals (speakers usually have springs, but screws are more secure). The notion itself is a bit weird, as a screwdriver is required to disassemble the headphones, and anywhere headphones work where a cable might need replacement (like, say, straight cable swapped for a coiled one), there's electricity for a soldering iron. It's also highly likely that whoever's going to replace the cable will also be familiar with soldering. Still, it's nice, and safe in a way as plastic inside the cups is not threatened with a hot iron.
 
There's a capacitor buffer before each driver, giving the ATH-M40 a slight curious sound mellowness reminding a bit of older synthesisers (CS-80, Prophet VS...).
 
The headphones were modded, and well... They weren't a major reassembly trouble like the ATH-M50 (there's a thread describing the assembly somewhere over here, with photos). Still, pushing through the new headband cable through was difficult.
 

Not the best photo, but there they are.
 
The new wiring is silver-plated copper, AWG 26, teflon-coated, with shielding (for the main cable anyway). The shielding is just aluminium foil. It all looks rather pretty inside the Techflex jacket, but foil isn't ideal as it tends to tear and deform over time, unless rolled fairly thick. A proper grounding copper braid would've been better, but it's heavier and it might be too thick to fit through the strain relief. Given a thinner braid though, it might also fit the headband cable (or better, two different diameter braids would have to be used).
 
The silver-plated copper cable cures the last stuck harmonics and veil the stock ATH-M40 have, and it makes them play full-speed, rather lively. The cable is terminated to a 3.5 mm. fat-barrel plug (the stock plug is 6.3 mm. full-size).
 
There's also Blu-tack on the driver assembly, helping stability a bit (the drivers are fairly stable as they are, attached to a platform assembly inside the cups), and Blu-tack, cotton and plastic fibre to improve isolation inside the cups. Isolation is important as these are street headphones and they're also used for monitoring next to loud drumkits and guitars. Studio/live work.
 
Impedance is 60-ohm, but the ATH-M40 are fairly easy to drive, an Apple Touch 4G player drives them to almost full noise isolation on the street. Not obnoxiously loud, but given the thing's low power, the ATH-M40 provide a stunning detail. They are a bit on the shouty/cold side when driven from proper headphone amps (and studio kit like mixers). What matters though is that they detect and show even very slight hiss and are suitable for track cleanup. As an example, in Nirvana's "Come As You Are" open reel tape noise can be heard even when playing on a weak player like the Touch 4G. Out of the sound interface, they show all the background noise in a microphone take.
 

Modified with silver-plated copper cable inside a heavy industrial copper braid shielding.

 
Sound-wise the ATH-M40 are transparent, they just put out what's there (tested against "straight-response" studio monitor speakers). Frequency response is not "perfect straight", of course, but they're just about right in that they show the picture. Response is fairly straight - compared to fairly straight monitor speakers (KRK RP6 G2 SE, they're mostly straight up to about 14 KHz), the ATH-M40 output is about the same, with more airiness and emphasis on higher midrange/treble (as is to be expected from monitor headphones, they're designed to pick out the dirt and show details). Electric guitars can be a bit light (and trebley) compared to what they'll sound like on speakers, but it's nowhere like the bodiless exaggeration of some other Asian makers (Pioneer SE-M290 - cold tint and bodiless guitars, Denon DJ DN-HP700 - too forward at 1.5 KHz, washing out electric guitars, etc.). Compared to speakers they're quicker (better dynamics and transients, though that's typical of any headphones) and they tend to be somewhat colder. As usual, it's not good practice to mix in headphones, so anything put together on the ATH-M40 will have to be tested and possibly rebuilt on monitor speakers.
 
The ATH-M40 were bought for monitoring next to loud sound, as the K-240 Studio are pretty much useless for that (being semi-open, they're like sieves when it comes to isolation onstage or even for studio tracking). Compared to the K-240 Studio, ATH-M40 are quicker, have a more "straight" frequency response (though this is not a flaw of the K-240 Studio, on the contrary, they're the best headphones for playing synthesisers and composing thanks to their tweaked frequency response) and have better isolation, though seriously, you ought to stick a few things like plied cotton inside the cups to improve isolation. Vibration killers like Blu-tack and Dynamat also help when used to pad the cups on the inside. Dynamics and overall painting finesse are better in the ATH-M40, but the K-240 Studio are still the synth tracking headphones that nothing can really beat, they're more musical (better note contrast) in spite of their being slower and darker.
 

Modded ATH-M40 and ATH-M2X (furry).
 
They're fairly comfortable, though the headband likes to slide around, at least the first couple weeks before it moulds and adapts to the head. ATH-M50 can start feeling crammed, especially for those with bigger/pointier ears, the ATH-M40 have more space inside the cups and give a nice, spacey soundstage as a result, though the drivers could sit closer to ears (SIGH, one of these days one ought to get custom-made and measured headphone cups  ).

Overall, they're a good buy, basically what you get is a headphone equivalent to straight-response monitor speakers for not as much (ATH-M40 sell for as little as $75 or so online). There're much worse headphones out there for twice the price, that's for sure. They work fine with anything, even weak cell phone outputs (just don't expect miraculous loudness there), so they're an automatic choice for all-purpose headphones. They're also fairly straightforward to mod, thanks to the wire screw-clamp terminals (the right cup has screw terminals for the headband cable as well).
 
Recommended.

AKG K-240 Studio are at least the 4th (really 5th or 6th?) iteration of K-240 design, which started with the K-240 Sextett in the 1970s, themselves a circumaural version of supra K-140. The basic idea behind the "Sextett" design is that the main diaphragm drives six passive radiators, which enhance ambience and spatial imaging. Now, while this is a very good idea for headphones in an era when most headphone amps were powerful enough to drive 1200-ohm and even 2400-ohm headphones, unfortunately modern amps have not been able to keep up and thus the K-240 and K-271 have a reputation of being power hogs. Really modern device (cell phones', pocket players', netbooks'...) headphone outputs are too weak and lame though. They're still efficient enough to be driven fine (thoughly slightly weak) by most Apple players.
 
Classic K-240 (Sextett, Monitor, DF) are all 600-ohm. The K-240 Studio are the modern 55-ohm version, lower impedance, higher efficiency. While they work with anything, really they have lost some of the smoothness of imaging and stability that 600-ohm offers. Still, they can be driven by pretty much anything nowadays. The only issue is that weaker pocket players and notebook PCs won't be able to drive them to fullness. They work just fine with any professional gear though, which is what they're meant for.
 
Make no mistake, the K-240 Studio are designed for use by musicians and sound engineers. Which is why they have a frequency response which is not straight by definition: midrange is raised to improve note contrast and show details in recorded takes, mixes and samples. It is not carelessly lifted though, and the K-240 still manage to have a very musical and rather natural sound signature, unlike their sibling the K-271, which has a weird treble/midrange transition and an overall melancholic character. The only advantage the K-271 Studio have over the K-240 is being fully closed; if you're trying to choose between the K-240 and K-271 for listening to music, go for the K-240, they are warmer and more natural.
 
Sometimes the K-240 are said to have a "sloppy bass", but this might be more an issue with amplification (the K-240 by design are not very efficient, having passive radiators that are driven by the main diaphragm too). The K-240 Studio are not designed as a boombox, but with the right amp they can deliver a lot of low-frequency power and massage ears with just about any kind of music, as long as it has low frequencies.
 
High frequencies can feel a tad too rolled off, but really this is a design decision as it seems AKG decided to roll some K-240DF properties into the Studio design too, bringing the overall frequency response closer to the DF model, while still keeping some HF response. As a result the K-240 Studio can sound darkish at times, but they're still sparkly and 3D.
 
Where the K-240 Studio really shine is space definition. As an example, stock-cabled Denon AH-D1000 will play a rock piece with the guitar in the centre, bass right next to it, and drums in the background, the singer floating in mid-air. In K-240 Studio, this same piece plays with the guitar in the centre-sides, bass separated better and tugged to the back, and drums panned all across the soundstage. The soundstage is quite large, and it's defined rather well to any side, even vertically. The K-240 like to separate instruments and pan them properly. This might also be an important quality for any classical music listeners.
 
The one big flaw of the K-240 Studio is the stock cable. Some say it's made of leftover old scrap copper, and to tell the truth, it sure sounds like that. The stock cable just won't make the K-240 show all they're capable of. Too dirty, too many stuck harmonics, too dark, too slow, bass and high frequencies slightly falling apart in the time domain, and of course, the delicate high frequencies get lost too, simply because the stock cable is not shielded. To make K-240 Studio show what there really is in a record, you need a good headphone amp (or output; a stationary stereo system amp will do, as will a mixer, a good pro sound interface, etc.) and a shielded high-quality cable. Shielded high-purity copper is one option, but they really become accurate with a shielded silver-plated copper cable.
 
Dynamics are slower than Denon/Foster headphones, but still quite good. K-240 aren't as fast, but they still paint with a good accuracy. They could be a bit sharper in dynamics, but with a silver or silver-plated copper speed/response is improved.
 
K-240 can be used for mixing, though really they're mostly for "zooming in" on parts of the soundscape and discerning details that are not as clear on speakers. They're very helpful when cleaning up samples and microphone takes. Like a "magnifying glass" for sound. They also work fine for monitoring. Don't let the "semi-open" bit fool you - even a singer in front of an LDC won't let music bleed into the microphone. A producer friend uses the K-240 Studio for singers in front of his C-414. They might bleed something when cranked up way too high (which is difficult), but in practice they're fine for any kind of studio work, though frankly closed headphones are still better for monitoring microphones, as they block more external noise. On the other hand, you can have radio talkers hear each other "live" and their own voice without taking off the headphones, which might be a downside or a plus.
 
The K-240 are built tough, from a dense plastic that's guaranteed to hold (mine once got bumped by a moving car). The "Studio" name is really appropriate, they're studio gear meant to last.
 
Overall, highly recommended. This is a must-have for anyone who plays synthesisers, edits sound, composes music, and mixes. Just recable

AKG GHS-1 have rather strangely made it here. As a gift.
 
They look like this:
 
 
 
And they're - wait for this - a gaming headset built around the K-81 core. There's no mistake whatsoever, the square apertures in the driver grille and the typical K-81 "bass, low midrange, midrange, some high frequencies" sound give them away.
 
Single-entry cable with a headphone volume wheel and microphone switch. The microphone itself is cardioid, and well, they work rather nicely for Skype, except that the microphone picks up too much ambient noise in public places. Tried them in Starbucks, and whenever the fellows from the right table said anything loud[ish], they talked into Skype pretty much. The microphone rejects the left side noise perfectly, but since the boom is fixed, and there's no way to swivel the microphone into a "facing mouth" position, it tends to pick up whatever comes from in front/right, slightly diagonal.
 
The velour pads actually worsen/muffle/bleed driver power. It's not that terrible, but they don't have the same impact as the K-81 unless you push them onto ears. The pads are puffed, so maybe tearing out some of the puffing foam will "deflate" them and improve the sound, sittng drivers closer to ears.
 
The upside, of course, is that they're comfier and softer on ears than the K-81.
 
The sound's nothing special, it's K-81 with no muddy bass (no bass boost foam), and with some power bleed due to velour pads and the drivers being farther away from ears. They're not really that great for music because of that, the sound gets a bit anemic with presence/power slightly dropped.
 
Overall the set can be summed up as "K-81 minus superbass foam plus microphone". And with velour pads instead of pleather. And with a single-entry cable. The headband is reminiscent of the K-2xx series, though there's a wire going through a plastic band, no steel wires there.
 
The headset is terminated with a single cable split in two at the end, both ends having three-pole 3.5 mm plugs. This was an issue for the original owner as his Apple Macintosh computer insisted on treating the microphone plug as line plug, never supplying power for the microphone. It works perfectly with the Roland UA-1G interface though.

Overview
 
First of all, you can find AH-D310 online for as little as $32, which makes them a very good deal. Possibly better than some $50-70 headphones, even. The "R" version has a microphone and IPod control, it's a bit more expensive, but worth it if you need that. So overall the headphones are a very good replacement for a mobile phone earset, for the money
 
AH-D310 are Denon's cheapest/lowest AH-D model, now that the AH-P372 have apparently been killed off. The OEM is, of course, Foster. The big difference with AH-P372 is, they don't fold into a ball, they fold flat (see attached photo). The other big difference is 42 mm. drivers instead of AH-P372 38 mm. drivers. In my view, 42 mm. drivers are a bit of a minus, as they don't have the balance between sweetness and power 38 mm. have. Just to give you an idea, AKG K-240 Sextett and first-edition K-141 headphones had 38 mm. drivers, and they're one of the "golden standards" on Head-Fi.
 

 
AH-D310 are obviously designed for weak players/mobile phones. Out of a Roland UA-1G USB interface, they tend to get overly bassy/boomy, but with a measly Nokia C3 phone they play great. So if you were looking for a headset, AH-D310R or AH-D510R might be just the thing.
 
Sound
 
The best way to describe AH-D310 is "supra-aural cheap sibling to AH-D1001". And Creative Aurvana Live, Denon AH-D1000, AH-D1100, etc. Fast trademark dynamics, great sensitivity (you can use them to check for hiss/background noises), nice detail. But less air/ambience than AH-D1001. Sounds like AH-D1001, but more congested, with a smaller soundstage, and with less sparkle. Soundstage is smaller than AH-D1001, obviously. Instrument separation is a tad worse. The problem here is, cups are too small, so there's not enough space for ears to produce enough "air" to the sound. They're also bassier. The curious bit is that apparently the headphones were designed for weak players, as they don't get to be that bassy with a weak headphone out, like that of a Nokia C3 phone.
 
As with almost all Asian headphones, AH-D310 aren't very fond of the third octave (~220 Hz), which is an issue for electric guitar music. The low midrange drop sucks out some detail and makes guitars sound emptyish. So you have to boost the 220 or 250 Hz slider on your player's EQ. Funnily, they work just fine with the phone (though this is after modding with Senn HD414 foam and a new cable).
 
So yes, midrange could use a bit more detail, but overall sound quality is pretty good, and the speed of Foster diaphragms makes AH-D310 play very lively. Bear in mind that this is talking more about the modded AH-D310; stock cable, as with all Denon headphones, doesn't do them justice.
 
They're not that great for playing music (as in, actually playing a synthesiser) as there's some midrange honkiness when midrange is overloaded with, say, virtual analogue (or real analogue) pads. Again, for weaker players and phones this isn't an issue as they simply don't have the power to overload AH-D310 to honk, but the sneaky suspicion is, that's refraction from the plastic driver grilles.
 
Recommended EQ for a more powerful player is: drop bass by one slider, boost low midrange (~250 Hz) by one slider.
 
Build & Comfort
 
AH-D310 are halfway between supra-aural and circumaural. They could be called "smallish circumaural", but they do not leave much air between the rear parts of ears and the drivers, which is responsible for the somewhat congested sound and lack of air in sound. AH-D1001, -2000, etc. all have larger cups with more space for ears, so ears get the full sound picture. Ears have to be fully exposed to the sound source for a realistic soundscape; cramming them into smallish cups denies some of the exposure.
 

 
Cups rotate on hinges and make the headphones fold flat (see photo). They look more fragile than they really are. Headband is metal with pleather/foam padding, and the whole setup is plastic.
 
Earpads are memory foam, and for the first couple weeks they might even hurt slightly on the ear rears, Later on they adjust and don't bother, but comfort-wise they're somewhere in-between supra- and circumaural too. AH-D310 might be large enough for a fragile Asian girl to call them "circumaural", but for a European male they're a tad too small.
 
Isolation
 
Not that much, actually. The cups are empty inside. No isolation materials or anything. They do attenuate external sounds, but with a weakish phone they'll freely pass traffic/people noise. With a more powerful player (or a sound interface like Roland UA-1G) they isolate by sheer loudness.
 
Modding
 
Mine have the full Mark L-style modding: silver-plated copper twisted pair (AWG 30) recable, Sennheiser HD414 foam to "sweeten out" the sound a bit (and boost midrange), Dynamat stabilisation, and cup filling. Which makes them pretty nice headphones actually, for not that much money. A lot warmer and livelier than any stock headphones, even those costing three times as much. They're the regular version, not "R", so no microphone or player control, but then it's not that needed.
 

 
One thing though: they can be hell to work with as the cup screws were driven in hard and deep at the factory. So the only way they could be extracted was with a screwdriver held with pliers. And that took hours.
 
Summary
 
Nice headphones for modding, in spite of dead-incrusted screws Also a good headset, though as it's wired for IPhones, it may not work with all mobile phones. The modded version is quite impressive, but then that's silver-plated wiring+Foster diaphragms. However, if you're interested in modded Foster headphones, then AH-D1000, AH-D1001, AH-D1100, and Creative Aurvana Live are a better value, as they're true circumaural and have more ambience/sparkle. Also more comfortable.
 
Long story short, this is a good buy. Recommended.