Does the UGB compress the signal? My thinking was no, the idea being that the UGB would bring the signal back to unity gain. However I've been listening to the steller DP's of Alpine '82. Garcia sounds fat, hairy, and wet, kinda like a strat into a dyna comp into a fender amp. Second thing is my 70's Vibrolux seems to achieve the same kind of thing. Very round sounding amp. Come to think of it lots of those pre coma shows have that kind of compressed sound, was Healy running the whole board through a rack compressor? If so I again marvel at that man's genus.
The UGB just changes the impedance ... it doesn't add compression.
Tube power amps will, by their nature, compress the sound when they are run hard. And there is the belief that Garcia's Mac was doing some natural and musical limiting when he pushed it hard.
I'm not aware of Healy using compression on the whole band, but I'm sure there were limiters in use as the transient peaks must have been intense through the system. And, of course, when the DP recordings were mastered, they were probably run through a multi-band compressor and had all sorts of other things happening to 'em.
Brad's probably the most knowledgeable on this subject ...