I don't own the Abyss but I own the HE-6 hardwired wit the Zeus cable. I was driving the HE-6 with a Threshold T50 (50 watts/ch Class A @ 8 ohms or about 8 watts/ch @ the 50 ohms of the HE-6) sounding exquisite. Now I am driving them with the MHA100 and to be honest I do not miss the T50 (which is actually for sale right now). I have tried them out of the headphones output with the high output and high impedance setting (that's what the guys from McIntosh recommended) and also out of the speakers tabs. To be honest I could not hear much of a difference. If any maybe out of the speakers tabs they have just a little tiny more body in the sound but it is really difficult to distinguish IMO.
I got the Abyss!
and used it w/ the MHA100 - using it w/ my Cavalli Liquid Gold now.
I personally didn't like the pairing of the Abyss and the MHA100 -
the Abyss, to me, have incredible bass response, its controlled and powerfully dynamic.
But there isn't much sparkly in the midrange, where most music lives!
The highs are nice too -
but I guess that also comes down to taste.
IT didn't come off as soulful to me as the Audezes or HD800 did...
I loved my Audeze LCD-2 w/ Fazor on the MHA100
LCD-3 w/ Fazor, LCD-XC, and my Senn HD800s!
Dug it with my Audio Technica 900X as well