I'd like to chime in on some of the stuff discussed in this thread. I own the Q701s and have owned the 880s.
In my opinion yes, the Q701s are shy in the lowest register (I feel they're good down to ~110-100 hz), but they're not as weak as some folks are suggesting.
2 things -- the 701s are very well defined and controlled in the lows despite the tapering off they suffer from. This may be a contributing factor to the fact that people are so far apart in their opinions of the performance. A low end that is well-defined although quiet, may actually sound more impressive to some listeners than say, a really big & full low end that lacks much resolution or dynamic range. This is not to say the two are mutually exclusive. It just happens that a lot of headphones in the 150-350 dollar range get it one way or the other, or that the cans that do strike a perfect low-end balance suffer from other quirks/shortcomings.
The other thing - as has been said a lot on the net, 701s take like a zillion hours to break in. And all that basically happens in the break in period is the bass wakes up some. At least that's how I perceived it. I bought my pair without having ever heard any of the AKG 60X/70X models, and I was considering sending them back after the first week... thankfully after like a month or more, they strengthened up.
All in all I think 701s are deservedly polarizing. You should buy them only if you're specifically looking for the qualities where they shine, which are dynamic response, stereo depth, super-fine resolution, and a mostly unaffected spectrum from the low mids to the top. I mix audio for a living so I really appreciate those qualities for helping me... but IMO they're not the most fun pair in the world if you don't require/desire the precision.
