I think it handles bass the best out of the 650/701/880 triumvirate because the bass is staged properly and stays very clear of other instruments while conveying pitch and texture. The bass is really one of my favorite things about the DT 880's presentation. It's not always the most precise but it never obfuscates other elements in the piece.
The Q701 doesn't blend bass into other instruments either, but its bass is much less full and not as satisfying. It's fairly light due to a lack of sub-bass. The DT 880 manages to avoid being bass-light while not being overbearing or muddy at all -- the bass just hits the spot. Cello and organs sound great compared to how they come off on the Q701.
Great... It seems like a brilliant option for the OP.
I would therefore go for DT880 (600ohm version) along with desktop O2 w/ 1x and 2.5x gain, RCA inputs and 1/4 headphone out + a better transformer (15 - 20v output, current does not matter that much). Current buffer mod is not needed for DT880.