I own the Senns 650's, AKG 701's, ATH W1000 & 5000's. Echoing the afore mentioned comments, the AKG's strengths lay in their nice detailed sound, wide soundstage and solid, but not over emphasized, bass. Decay is very good and the AKG's can handle many layers of music as 1markr pointed out.
Hififile is spot on that the 701's are, by comparison, not that efficient. JPStereo stated that a good powerful amp is necessary to get the best out of the 701's and my experience bears this out. My 701's are currently paired the Cayin HA-1A most of the time. As this tube amp has a lot of grunt, it exhibits good control over the 701's and has a natural synergy with the tonal qualities exhibited by the 701's. The Singlepower mentioned above would also be a very good choice to get the job done.
With that said, I brought them to work today and listened to them nearly all day paired with my new Corda Opera (SS) and was very pleasantly surprised - especially with respect to bass control. Mid and lower bass were nice and tight and the mids and highs had great detail as well. The Opera is the first SS I've heard that seems to have a hint of a tube warmth. If your tastes lean more toward a SS sound that is not overally analytical, my bet is that you would indeed be happy with the Opera / 701 combo.
For my taste, I do believe that the 701's do fare a bit better with tubes - YMMV.
Keep us posted.