Beyers, well used Beyers, this is a bit gross, but there is a very good chance, if they were used a moderate amount, even if they appear well taken care of, that there is hair all over... and poking into/at the driver inside the headphone's earcups. The hair pokes through the black foam into the cups, and it gets stuck in there. It doesn't help that the surface of a Beyer driver is kindof sticky.
I had a used set of DT990 pros (same construction as far as hair would be concerned) have this, they looked fine, but there was so much hair on the drivers, they were a write off. I tried to use tweezers to get it out, but there was so much hair, and so much was embedded in the driver membrane, that the drivers were destroyed. They made a crackling sound with higher levels of bass. It certainly wan't a lack of amp power. (I was using a Grace 901 ref. amp)
I could be wrong, in fact, I want to be wrong. It's gross... but also likely depending on how used they are, and the owner's hair type/length.
On another note, the Sony XB series has two very decent headphones: the XB700 and XB1000. These are basshead cans with the finesse to handle the rest of the audio band with proper respect. I *really* like the XB700, and I would place it as a better sounding headpone than the DT770pro/80 in many ways. It is a bit warmer, which can be good or bad, but it doesn't have the resonance and damping issues the 770pros have. The XB series does sound better on lower power systems than the Beyers. Some say the XB1000 is an improvement on the 700 all the way around, but the 700, to me, is where it's at. It's also at a good price point.
The XB500 is popular too, and some will say they like it more. Generally, it is more difficult to get more than just massive bass out of the XB500. It seems to take a knowledge of EQ to bring out some of the qualities the 700 and 1000 already have, and since EQ can degrade the signal quality, the best that can be done is still at a loss. Unless the 700 is too expensive, there is very little reason to consider the xb500 IMO.
Just be wary of how "lightly used" those DT770s are. People on ebay lie... constantly...
I mean, the 770pro is no slouch when it comes to bass, it is just a bit boomy, and doesn't have the control that other headphones can have when it comes to the low notes. I agree that you would see an improvement with a better amp, but honestly, I think the Cmoy could drive the 770pros well enough. I use them sometimes with an extremely low powered tube amp (it might even have less output than a cmoy), and unlike the DT880pros which sound like crap with such low power, the 770pro/80s have a decent full sound, and while I can't crank it up terribly loud, it doesn't sound bad. I get more than "one note bass" anyhow. It does take better associated equipment to get a wide "headstage" with the 770pros. The XB700s seem to be able to project sounds outward around the head a bit farther on less money, if this makes sense. Ultimately, the 770pro is going to be able to do this better, but it takes more expensive gear to do it. I was impressed with how open the XB headphones could be. You really don't expect them to sound all that good when you put eyes on their blister packaging with EXTRA BASS written all over it in the store.
Straight out of my Sansa, the XB700s win easily enough. Something to keep in mind if you just want to take a portable and a headphone and nothing else. XB series is really a good deal for budget minded bassheads, plus they have them at stores you can walk into, which can't be said of some of the other bass recommendations. The nature of the modern basshead leaves little room for the DT770pro/80, but it used to be a top recommendation in this regard.