There are other versions of these amp PCBs, the 638 & 640, which use ICs driving a comp. pair transistor output. Interestingly enough, I have swapped them in 31105 Mic Pre/EQs in the 8078s that I used to take care of, and NO ONE could tell the difference.
My friend Fred Hill in Nashville maintains that the sound is in the XFMRs, and I tend to agree.
From what I know, and the consoles I have worked on, the 6xx series were designed for the 8088 series of consoles, which are large frame 8068s, using 31102 Mic Pre/EQs. These were the consoles built for the AIR studios, that wound up at A&M/Henson, Bryan Adams studio, and a few other places.
I currently take care of a lovely 40 ch. 8088 in Burbank, CA.