[jac3atx]

Another head-fier and I did a head to head comparison of his K701's and my DT990~600ohm on Monday using my Woo 3+ amp. In terms of comfort, we both agreed the the DT990's were more comfortable because of the softer pads. We both agree that the K701's look completely awesome (which sadly doesn't impact the sound). The only genre for which I would prefer the K701's is classical and maybe some acoustic stuff (and he didn't prefer the K701's for anything -- the grass is always greener I guess). Switching back and forth on some acoustic guitar tracks by people like Kaki King or Al Petteway & Amy White, I found the DT990's a little warm and "syrupy" if that makes sense, BUT with some very subtle EQing down of the hump between about 100Hz and 400Hz I found that the syrupy-ness went away and everything felt crisp and precise. I do mean subtle too -- you don't want to turn the DT990's into the DT880's, you just don't want the mid-bass hump to pollute the mids themselves. After proper burn-in, I have never felt the need to decrease the highs via EQ (before burn-in was a different story).
 
I hope this helps. Either the K701's or the DT990's in 250 or 600 ohm will demand amplification to get your money's worth, but they'll certainly want different amps. Unless you are all about classical or already have an awesome solid-state amp designed for low impedance cans, I would definitely vote for the DT990~600's.