From a post of mine:
I realized something about the HD650, now that I'm listening to my 2nd pair (rebought ~6 years after selling my 1st pair): an immature listener might completely under-appreciate what they do, how they present recordings, have no idea and then just sell them like I did. It took me that many years to understand what I want and what I hear... It's hard to explain but at the moment I feel like these bastards will only be sold if I go deaf. They're just so competent, they shift my attention to music. The detail level is really good and the way they image is sorta enigmatic but competent and engaging. I can't say that I miss the soundstage or sub-bass of my bass-port modified Q701. It's like when you listen to HD650 you're in a world where other headphones don't exist, or the things that HD650 are incapable of are just irrelevant or don't matter. This second pair smells exactly the same as my 1st (I wonder what psyche-manipulating stuff Senn puts into that fragrance...).

BTW I think it's important to maintain the pads in good shape, replace them once they compress and especially replace them right away on a newly acquired used pair (mine is ~12 years old).

Modded 250Ohm version sounds really nice, I just put them on after keeping in storage because of ATH-R70x.

I can definitely see how this could be an end-game headphone for some people, a retirement headset, as someone has mentioned around here. They're durable (passed the sit-on test) while being capable of very satisfying and pleasant rendition of music that is probably quite close to how it should sound. The music is presented as a beautiful whole, a full performance where every instrument is just there in it's place both tonally and in the sound-stage. This could be a retirement headphone literally, because of the slightly sparkly treble, just a little top-end sparkle. Vocals sound great, there's a slight emphasis on them compared to ATH-R70x which is really good. A weird thing to say, but DT250-250 sound earthy, organic and natural to me. When you put these on, you hear music, a performance, not details; separate parts don't call for attention. They're like here's your music bro, chill out. Not suitable for commuting as they isolate poorly in the lower frequency region.

My personal recommendation in the price range for their sound.

Edit :: 2016-04-28:
Same impressions when returning to them. They bring every note and nuance in music that it sounds like it provides the 'whole picture' without focusing on unimportant details.

Edit :: 2019-01-14:
My review still stands. I've modified this headphone since purchase, put some acoustic foam and sealed the input socket. Kick drums sound big, quick, impactful, punchy - this headphone has a "big butt" to its sound. But it's hard to tell apart the lowest notes when playing my synth - A1 and B1 notes sound unacceptably similar. So the sub-bass resolution is lower than my 600ohm DT990, Mayflower T50RP mk3 v1 and modded mdr-ma900. This might be fixable, just need to refine the mod, maybe add a bass port or completely close off the headphone, not sure...

200 bucks?!

In short, these are the strongest performers I've heard and they made me sell my HD650 very quickly. I did enjoy the HD650s with some recordings, mainly vocal music, but they were lacking some breath, air and energy to me, while Q701 delivers all that and more. HD650 were like visiting your grandma -- fuzzy, warm and cozy, and Q701 is a lively party -- everything is lively, accurate and no detail is ever left out. They have a very nice impact. Every drum hit is very satisfying. Oh and the soundstage...why repeat ourselves; it's been said enough times already and I guess it's so good because of the angled ear pads. Also they don't choke no matter how intense your music is. That is, they are really fast sounding; fast rock music on Beyerdynamic DT250/HD650 sounds a bit dull and details/instruments get lost in a mess. Q701 presents every part of the music for you to focus on. And they sound so lively and clear. Choir and classical sounds truly impressive. The upper mids are sometimes a bit too loud with modern hotly mastered music. Though that never gets in the way with proper recordings, ever. Be gentle with them though. Those clear plastic parts in the headband tend to break easily. If that happens, cups won't be sliding as smoothly when adjusting and may start creaking. So yeah, don't sit on them.
 
 

Bass port mod

Mind you, all my impressions are after doing the bass port mod. I don't remember how they sound stock. Some say it's the same, just without the sub-bass. Yeah, sub-bass -- it goes very low and is another star of the show, fellas. Tightest and most accurate bass I've heard. HD650 bass was anemic compared to these. I never use EQ. The pair I have was made in Austria.
 
 

Cup placement!

If these headphones would automatically adjust to their best acoustic position around your ears -- they would be perfect and their fragility could be forgiven. I used to push the cups towards my face, until they touched my helix. This placement is far from ideal, as I later discovered. For me it's best to start from placing them all the way towards the back of my head, until I feel the cups touch my tragus. And then push them just a bit towards my face, until the cymbals sound right and my ears end up somewhere in the middle of the cups. After that is achieved, I'm presented with the best sound. Otherwise they sound quite wrong.

 

Highly recommended

If anyone asked to recommend a desktop set-up, this would be it: ODAC->O2 Amp->Q701[+ bass port mod]
If they don't need isolation, this setup could easily be the most they would ever need, and for a great value.
I rather like this one, while I myself have the generic ODAC + a DIY O2.

So

If you're feeling adventurous, try these headphones. No, I mean try them. Screw that, just get them now. I could keep adding impressions. But that's really starting to get in the way of listening to them right now. There's really nothing to lose, they're $200 on Amazon, which is crazy.