I will probably write more... but I tried out my M-Audio Q40's today, and I really do like them.
Here is the short version:
Tried them first unamped. They sounded great. Easily as good as Beats Pro (from what I can remember). The bass was very nice, and nothing else was missing or muffled.
Then added the Fiio E6 and LOD, and wow! The bass came alive and these are now easily the best headphones I have ever heard... (though I've never tried anything really expensive).
The bass definitely goes very deep into the sub-bass realm, just as I was hoping for. The bass is definitely not overpowered for my taste... in fact I was assuming it would be a little more forward than it is... maybe why so many people like pairing these up with a bass-enhancing amp.
Overall, huge thumbs up, and thanks to everyone for helping me make this decision.
I did a quick comparison with my other cans: Q40 vs. XB500: No contest, the Q40 wins in every category except maybe the amount of mid-bass... but I still call that a win for the Q40 since the XBs have probably too much mid-bass.
Q40 vs. Sennheiser HD570: HD570 wins for comfort, but they are open-air, light, and have velour pads, so it's not fair. I have velour pads in the mail already for my Q40s. Sound quality-wise, the HD570s are more airy (duh) so therefore have wider soundstage, but I can hear more detail and accuracy in the Q40s easily... and bass-wise there is no contest. The HD570s have bass, but they are not basshead cans so they are not even in the same ballpark for bass and sub-bass.