a well dialed in 1266 TC is indeed hard to beat. I run mine using a tube pre and class D amp combo and love it. i use an EQ (have Mitch filters but prefer my own) and it solves the upper mids "honkiness", reins in the highs, and produces insane bass slam/punch and sub-bass rumbles.
I have the immanis as well; it's a great all rounder with added comfort. it's less picky on amps/source and much much less finicky with fit. I also run an EQ with the Immanis (mostly just a low shelf for added bass, but occasionally a high shelf for some music which is a bit brighter or energetic up top).
head to head, the Abyss sounds bigger and grander and doesn't lose out much to immanis except perhaps in the mid range; with immanis, vocals are a step or two more forward and sound sweeter, with more accurate tone and timbre imo.
to me, Abyss feels like being in the middle of a concert. Immanis feels like being with the artist in a room without borders. both are great headphones. if I anticipate more than an hour of listening time, or at the end of a long work day, I usually reach for the immanis for the comfort. if I have less than an hour, I'd probably reach for the abyss just for the sheer fun and impressiveness of the sound, comfort being less of a factor.