ok.. on v2 triaxis' the first ones had a tx4 recto board (ld1red), some said it was too bright, so mesa offered a phat (fat) mod to address this. This can be added or removed (i removed mine).
my take is, ld1red without phat mod is tight, and chunky on the low end. With the mod, it loosens up that tight chunk crispness, mudding it up some, i can see it might be usefully if you are not a high gain person. personally i hated the phat mod, so i reversed it.
some confuse the phat mod with the newer triaxis' which have a new ld1red board called the tx5, this is a TOTALLY different sound all together. so now with v1, v2, v2phat, and v3 (some calling it that) you have 4 different sounds on the ld1red channel possible, so not everyones opinion of this channel will be the same.
Dave