Another Head-Fier here helped me get a pair of these from the Koss outlet store recently. On day one I didn't like them at all. Had this very weird and unbalanced sound signature. When using them with games it's very noticeable where it's focus is. After day one I found they just had too little bass and way too much treble for me. Tried them later and had the same impression.
 
I have to say, these are growing on me fast. I connected them up to an E9 (!) with an HRT Music Streamer II and they sound much better.
 
It's close to being bass light and some sub-bass is just not there and seems to go missing. Not a big deal for me. Just not as present as on my DJ100. The mids are kind of thin sounding, but a little more forward than that of the DT-880. These sure are aggressive in the treble! More so than any other headphone I've heard. Just don't turn them up too loud and it's not so bad.
 
The sound from these is crystal clear. They're perhaps one of the most detailed headphones i've ever heard. If you want to be impressed, try these with classical music. For being closed, the soundstage is quite huge! Probably even larger than the K550 depending on the track.
 
I can see how many people would either love this headphone or hate it. With some music it's very impressive and on others, not so much. Despite it's good sound, I wouldn't call it balanced. I think it's sound clarity is up there with the SR-325is. No joke.
 
Comfort isn't bad and the pads are massive! The build quality is pretty poor for something that once cost $250. I imagine most of that went into the sound, but it did have a lifetime warranty. I bet if this died, they'd still honor the warranty and give me something else!
 
I bet these would sound best with a warm amp/dac. I'm using a Canare interconnect cable and that seems to help to tame the treble a tad and improve the mids a little.
 
I only have maybe 10 hours on these so far, so I'll edit this review if anything changes.
 
Closest match to this signature i've heard is maybe the DT-880 or maybe the Grado R-325is, but with less mid-bass.
 
 

I'm not really a fan of computer DACs and prefer full sized CD players mostly. I previously had a Nuforce Icon Mobile DAC and it's ok. I believe it's the same thing as the uDAC, but I might be wrong. It seemed to be a good match for only specific headphones and I did prefer it to the E7 by far. It's very good for bass and treble happy headphones. It's especially good for the M50 and D2000. Despite poor measurements it doesn't sound that bad.
 
Once I got my Q701 the first thing I wanted to do was get a good dac. Nothing overkill and something that's a good value for the money. I'm not so serious about music to need a $300+ DAC (yet). I actually figured there would be a very slight improvement in sound quality. Boy, was I ever wrong.
 
For me, this DAC has been a bigger improvement than upgrading Amps. No joke. This really seems to improve the level of detail from all my headphones. Pretty impressive. The bass was much improved on the Q701 (didn't really add quantity, maybe a tiny bit) and the soundstage seemed to get a little larger.
 
I've heard before that this DAC might have "edgy treble", but that couldn't be any further from the truth. The treble is perfectly smooth as long as the song isn't badly mastered and sounding like garbage to begin with. It won't suddenly make any treble happy headphone worse.
 
I didn't review this right away because I wanted to figure out it's entire sound signature. I do think it's pretty much close to being neutral and won't magically change a headphone's signature. I do think it is slightly warm. It won't suddenly turn a DT-880 into an HD-650, but it does help some thin sounding headphones or those lacking warmth. I've found it especially good for my SR-325is, Q701 and DJ100.
 
I do think it MIGHT have a very, very tiny peak in the upper mids somewhere. I noticed this only with the K601 and K400. Strangely enough I upgraded my firmware and it seems to have gone away. This would be impossible right? Probably placebo. Could be just revealing to me how bad some songs are, who knows.
 
This seems to really improve many, many headphones. It's kind of ridiculous. You'd be shocked to see what this does for the KRK KNS-8400. You think the KRKs are detail monsters from a portable player? Waiting until you hear it with this. It's as if it's trying to suck every last piece of detail out of your music file and send it to your KRK. With the KNS-8400 it seems to improve the soundstage size and level of detail. Kind of add a little bit of texture to the mids and make it a tiny bit warmer sounding. Overall, just a little bit more fun to listen to.
 
The absolute BEST match I've had was with the AKG K601. It doesn't need to be any warmer than it already is, but somehow this DAC seemed to breath new life into my K601 that's been in storage for months. Feels almost like I got my K601 upgraded.
 
Here's another crazy thing to annoy the science guys. I never could hear the difference between interconnect cables at all EVER. When I upgraded my DAC to this, it's stupid easy. No, not USB cables silly. I still use cheap cables, but it's funny hearing the difference between a Monoprice cable and a Canare cable. I actually prefer Monoprice, but only due to preferences.
 
Strangely enough I like this DAC less with my HD-598 than any of my other headphones. Not sure why. Maybe it's that my HD-598 is already as warm as I want it and it does not benefit as much. Who knows. I have a feeling this would be a bad match for say the HD-650, but I'm not sure.
 
I feel that it's totally worth the $150. I'm using it with a Headroom Micro Amp and it seems to be a perfect match.
 
8/15/2012 UPDATE:
 
I've decided to part with this DAC. It's still a good DAC, but too forward sounding with my Q701 and K601. Often a tad too fatiguing. I've even compared it vs full sized cd players and soundcards and those were much more neutral sounding. It's not a case of poor recordings.
 
The soundstage is also much too small. I blamed my amp at first, but was I ever wrong. Even my full sized CD player has a larger soundstage.
 
I'll probably use the funds to buy an ODAC or a good soundcard.
 

Last year I scored these at a local Wal-Mart for only $36! What a deal. I actually expected them to sound like my AD700 somewhat, but they most definitely don't! Not even close really.
 
I was really surprised by how much I liked these. First, they are smaller than the AD700 and have a more secure fit. The AD700 is a poor fit for me and are much too lose. These are much better for those with a smaller head.
 
These have much more bass than the AD700, but are not bass heavy or even close to it. They have very full sounding mids and have a very warm sound (not like a Sennheiser!). I was pretty surprised about this since I always felt the AD700's mids to be fairly lean and kind of just not doing it for me. Overall, just so much fuller sounding mids on the AD300 compared to the AD700. They are fairly forward and quite engaging to me. I remember listening to these for a very long time the day I got them.
 
The AD300 also doesn't have the treble of the AD700, so it feels much more balanced and smooth sounding than the AD700. It doesn't have the weird tonality of the AD700.
 
The soundstage in most recordings is MUCH smaller than that of the AD700. It's still good, but not massive at all. These are especially good with female vocals.
 
The weakest part about this headphone is that the sound clarity is good, but not great. It's about comparable to say the old HD-555 perhaps. They have a good amount of detail, but these are fairly forgiving. I don't find them muffled at all though. Imagine a mix between the HD-555 and the AD700 perhaps.
 
They run around $65 right now and for me it's totally worth it, but it depends on the sound signature you like.
 
BTW I wouldn't be surprised if these had more bass than the AD900! I haven't heard the AD900, but it'd be interesting to know.

This is a review of a fully modded UR-55. Out of the box it's good, but not great. Without the mod the mids are very slightly muffled with some songs and the bass can be bloated. All this is caused by the grill and fabric behind it. I'm 100% sure of this since I had two pairs to test the differences!
 
When modded and these two things are removed the sound improvements are just amazing. Nothing subtle here in any way. Everything is more clear, the bass is not bloated and more controlled and the soundstage is much better. I think this is a driver that needs a lot of air to sound it's best.
 
Basically the only way I can describe the signature is just something that sounds "just right" to my ears. It'd be neutral if it wasn't for that slight bass boost to make them a bit more fun to listen to. The mids don't get drowned out and vocals are crystal clear at all times. The highs may be very slightly rolled off, but I find this a good thing.
 
The level of detail is quite high, but not as high as the Koss Pro DJ100. I think once modded these would make a PERFECT budget gaming headphone. No joke! They're that good. Strangely these are one of the first $50 headphones i've ever had where 128kbps MP3 files sounded like garbage and I could even hear recording hiss in some of my songs.
 
Before the mod, my Maxell DHP-II was better, but now these are my favorite un-amped headphone under $80. I actually bought the Shure SRH-440 and they lost out to these (I do love the SRH-840 though). The only thing that's comparable in sound that I own (under $100 and un-amped) is my modded Grado SR-80. The sound on those is slightly more clear and it's a brighter headphone, but does lose out compared to the UR-55 in some areas. Modded UR-55 has more bass, a better soundstage and imaging and better comfort. Mids may be slightly more forward on the SR-80. The SR-80 thought is still probably better for metal, but for $50 it's hard to complain!
 
More people should try this mod. This is really one hidden gem in the headphone world. It's only a slightly above average headphone without the mod.
 
Anyone who loves the Porta Pro or KSC75 should love this headphone when modded!

I've tried about two dozen pairs of headphones and I'm always looking for the absolute best pair for vocals. I also want forward sounding mids, but not too bad. My preference is also for highs that are not harsh or fatiguing. Being somewhat rolled off is ok. I find the SRH-840 is what I like in this area.
 
The best thing about this headphone is the quality of vocals. It just sounds amazing with both male and female vocals. This headphone is absolutely perfect for Jpop, Cantopop, Imogen Heap and Radiohead. The strange thing about this headphone is that the sound feels like an IEM. You feel like you're right there.
 
The highs are somewhat rolled off, but not as badly as the HD-600, but slightly worse then the k240 (55om) and SRH-840. This and the overall sound signature makes them a VERY non fatiguing headphone in every way.
 
Overall, this is a very neutral souding headphone. The only exception is that the mids are slightly forward, but not too badly. About the same as the Grado SR-80 or a tad more.
 
Here's the strange thing. Koss labels them as having "extreme" bass. They sound like they have a lot more bass out of the box, but after burn-in it gets reduced. I don't find these to be bass light or anywhere near that. The bass is VERY well controlled and perhaps people mistake them for being bass light because of this. It has just slightly less bass then my burned in k240 on an amp. Don't buy these if you like a ton of bass. Overall I find they have about the same amount of bass as the SRH-840 or maybe slightly less.
 
Second best thing about these is how natural and accurate everything sounds. Basically, everything sounds pretty similar to what they sound like on my HD-600. These are extremely picky about the source quality. They're VERY revealing of lower bit-rate and poorly mastered CDs. With my HD-600 I feel they can make anything sound good.
 
Soundstage is very good for a closed headphone, but not great. What really impressed me is the imaging of these headphones. I basically feel as if I can pinpoint every single tiny thing. Even more impressive then my HD-600. I think it has to do with the closed design, but i'm not sure. My k240 has fairly poor imaging and that's semi-open. Instrument separation is also very good.
 
The problem with these headphones is that they NEED an amp. I use them with my E5 and Nuforce mobile and they don't sound as good. Despite the low ohm rating I don't suggest them without a GOOD portable amp. I use them with my Total Airhead and they sound great! With a good amp they also have a bit more bass, but not much.
 
Another negative is that my first pair broke due to my own fault. I bought another and they sounded bad out of the box. The mids were very recessed and they just sounded wrong. The mids are nowhere near being recessed normally. I burned them in and after the 1st night they sounded better, but not quite. On the 3rd night they were perfect and just like my last pair. My first pair didn't seem to need burn-in to sound good.
 
I also found out these work with the Pearstone Velour pads. They need to be stretched out though and they look a bit goofy, but not too bad. They're now extremely comfortable. Out of the box I gave the comfort a 7.5/10, but now it's a 9.5/10! Luckily they don't degrade the sound quality.
 
Another nice thing about these is that they're extremely small. I think they're only 6.5oz without the cable. The build quality is also excellent. The cups are 50% metal along with the internal band and one other part.
 
Two other headphones sound very similar in some ways. These sound almost like the Maxell DHP-II, but those are far less neutral and have more bass and mid-bass. The vocals and mids on the DJ100 are far better. The build quality on the DHP-II is also not good. I'm almost positive they share the same drivers or something close. For anyone that cares, Koss has told me that they drivers on the DJ100 are titanium. They're 40mm when I checked my pair.
 
The Koss Pro3aat also sounds similar, but has way too much bass for me and too much clamping force.
 
These headphones are not for everyone. If you focus on vocal quality and good mids, get these. If you want sparkly and very bright highs or massive amounts of bass, look elsewhere. If you love any kind of Jpop or Cantopop, these are an absolute 100% must have. I would also skip these for rap, classical or metal. Classical isn't bad on them. My k240 are far better for classical.
 
My 2nd favorite is the k240 Studio. Those are better for more genres, but the vocals are not as forward and clear. The overall level of detail is also just not as forward. For me, the DJ100's level of detail is a step down from the DT-880 and AD700, but not by much. The mids are like my Grado SR-80 in some ways but without the fatiguing highs (un-modded). The DJ100 has far more bass though, but less mid-bass.
 
So, as you couldn't tell, this is my favorite headphone i've ever owned!

Back in 2000-2001 I used a cheap pair of Philips earbuds during exercise with just a CD player. Back then I had no mp3 player. They were the types that had an adjustable ear clip and they had the perfect fit even with glasses. I liked them so much that when they finally broke I bought the same exact pair! There must have been SOMETHING I liked about them. Obviously I upgraded to something better and have not went back.
 
The other day I was at Wal-Mart and saw a cheap pair of Philips $9 earbuds. I kind of wanted to try them out to see how they sounded. I was just curious. They were nearly the same as the old version.
 
I was going to just not even open them because I felt like it'd be a waste of time. Opening the box I was kind of laughing because I felt like these were going to sound like garbage and that i'd be wasting my time.
 
I hooked them up to my Nuforce Mobile headphone amp (totally not needed) and they sound actually pretty decent, maybe even kind of good! For a $10 earbud that is! The sound out of them is actually very clear and detailed. The bass is almost non-existant despite them having some "Bass Port" as a feature. You certainly don't feel ANY bass at all. I listened to my favorite reference track and one thing I always listened for was still there, but was nearly missing. It sounds like a fireplace bellow(?) and is kind of hard to hear. You feel it on some high end headphones, but not in the cheap stuff.
 
So, why would someone use these? If you want decent sound quality for a maximum of $20, these would be one option. They don't sound anywhere near as good as the Koss KSC75, but those are MUCH more bulky and not good for ALL exercise. These headphones would be good for a runner that wants some decent earbuds with a clip and doesn't want to pay over $25. For very extended listening, obviously i'd get something else.
 
BTW these have been in my ears for the last hour and it feels like I have absolutely nothing on my ears. They're just so light. They fit even fine for me even with glasses on.
 
I haven't tried many other cheap earbuds lately so I can't compare them to much. I do feel these were better then the $20 JVC Flats headphones I tried. I really found those to have very poor sound quality and I took them back.