Price for the hd650 and q701s seem a bit overpriced, for the dt880s the price seems ok. By the way I dont own either of these headphones so its just my impression after hours and hours of research that I've done.
None of those headphones are closed so they all will leak quite a bit of sound. The HD 650s are the most expensive and rightfully so, they are in another league. For an open headphone they have very good bass with a sound tilted to the warmer sound of neutral. They have the 'Sennheiser sound' which people may say is laid back but non fatiguing; some people hate it. These are 300 Ohms.
They DT880s are semi-open, but there are a lot of different version of the 880s. There's the 32, 250, 600 Ohms versions of the 880s. All are relatively the same I think except the amount of juice they need. I hear its also has very good bass.
Didn't research the q701s much but I hear they are a bit bass light. Have really good mids and highs, but it can get a little shrill and in your face after a bit of listening. I believe they are 65 Ohms.
The E7 has a weak amplifier so you can squeeze by with the Q701s and the 32 Ohms version of DT880s. For all the rest, the E7 will barely have enough juice to power the headphones. You'll probably need to get the E7+E9 combo which should power the others fine.
Edited by ztreb185 - 12/4/11 at 10:19pm