Well, here's an update. I think something was wrong with the K701's I had, and I ended up selling them and picking up a pair of new AKG K702's, which have the nice detachable cable but otherwise appear to be the exact same phones. Quite oddly, however, they sound completely different than the K701's I had before, which leads me to believe that I had a bad pair. There's plenty of bass, even with only 30 or 40 hours on them, and the amount of layering and separation I hear is amazing. They are easily my favorite headphones so far, and appear to fall somewhere between a Grado and HD650 in terms of sound. I look forward to hearing what sort of difference further break-in will cause.
Also, I have tried these on the Crack with a Speedball, film caps, a 12BH7 driver, and the usual 6AS7 in the power tube position. Interestingly, I hear little to complain about, and the combo still sounds better than the headphones being driven from a solid-state amp. I think the device driving the amp (Music Streamer II), which is sort of dark-sounding and loud, does help the situation, as does the 12BH7-EH, which is also a bit dark-sounding. If anything, the AKG's are simply are a bit loose and weak in the bass, but it's too early to tell if that is impedance-related or simply an artifact of their newness. My feeling is that a 5998 would solve most of the bass issue, but I (stupidly) sold mine before getting these new K702's....