I am a reasonably long-time owner of all 3 headphones DT880 (mine are 600ohm), K701, and HD600. I would encourage you (rather than quickly swapping the headphones) to actually spend a day with each - and then see if your ideas on the sound of each actually change.
You may find that after spending longer time with a headphone like the HD600, and only the HD600 - then switching to the Beyer or AKG - that you will then change your mind on which is most balanced. Instead of the HD600 being overly warm and lacking energy - you may find that they Beyers / AKGs instead sound overly bright. It's an interesting exercise. Our brains adjust to what they are used to - then compare other sounds to that base. As the base changes - so do our impressions of a particular headphone.
Anyway - it's a good idea for a thread - I'll also throw in my quick impressions of all 3. My set-up is Foobar (PC) > optical out to an Audio-gd NFB-12, then either direct to the headphones or via my LD MKIV (the amp doesn't change the contrast, nor my impression of each).
DT880.
Quite flat - but with elevated high-end. Bass extends pretty well and is nicely defined. Mids appear slightly further back in the spectrum - particularly compared to the highs. For me - it is the most detailed (like shining a spotlight on the music). Has the smallest sound-stage of the 3 - but still great for almost any genre. Very, very comfortable. Definitely brighter than 'normal' sound.
K701
Again reasonably flat response - but with more emphasis on mid-range and highs. Bass is recessed compared to mid-range and highs - but what is there is well defined, and IMO sounds really good. Definitely the most bass-light of the 3. Does a beautiful job of the mid-range - particularly acoustic and stringed instruments in general. Widest soundstage of the 3 - by quite a margin. Imaging is OK - sometimes struggles with center stage (not sure if that is particular recordings though). Handles fast paced music pretty well. Appears very detailed - but IMO the Beyer shows more actual detail in comparison. Comfort because of the bumps in the headband is not great. Because of this, I removed the bumps, added a small layer of foam, and then finished it with a soft lamb-skin leather layer. It is now extremely comfortable.
HD600
Whilst I wouldn't call it exactly flat - I would say it gives (to my ears) the most natural presentation of the three. It is the most life-like and has really good resolution and timbre. Sound stage is close to the Beyers (marginally larger), but the imaging is better than both to me. Has a little mid-bass bump, and gives the impression of slightly forward mids. Appears slower than both the Beyer and AKG. Appearance of less detail is often misleading - the detail is all there - it just doesn't have a spotlight shone on it like the others. Comfort is pretty good - although a bit clampy at times. Over time they have become more comfortable for me - and sit between the Beyer and my (now modified) AKG.
Of the three headphones - with a good (well recorded) live performance - all three headphones sound superb - but the HD600 is the only one (if I close my eyes) that can actually give me the feeling that I am actually sitting in the audience - their tone, timbre and imaging really is superb.
Short summary (just my view):
Bass (quantity) - HD600 > DT880 > K701
Bass (quality) - DT880 > HD600 = K701 (just different presentation between the Senn and AKG)
Mid-range - HD600 > K701 > DT880
Highs (quantity) - DT880 > K701 > HD600
Sound-stage (size) - K701 > HD600 > DT880
Imaging - HD600 > DT880 =/> K701 (tough to call depends on recording)
Naturalness - HD600 >>>> K701 > DT880
Comfort - DT880 > HD600 > K701
I love all 3 headphones, and I probably spend equal time with each - depends on my mood. They are all stellar performers for their price.
