I think it's the basic Dynaco circuit that imparts the Bass friendly attitude that Greg mentions, and also the cleanliness and articulation that Isaac posts about.
It's a Hi-Fi amplifier, distortion was the
enemy, something to be avoided.
As Isaac mentions, the Sundholm era Sunns take pedals very well, something that Reeves era Hiwatts also do well, and for the same reasons IMO. It was a different time. Both used the tube technology of their eras to create an amplifier that, well, just
amplified accurately rather than get all nasty fast.
Even Fender had the same approach; I've heard from what I consider reliable sources that the reason Fender went to the "Blackface" amp circuit from the now very sought after "Tweed" amps was because the Tweeds were too "dirty" tonally for the higher powered needs of the 60's.
I agree with Greg; Sunn has always been more identified with Bass, and are
still revered in the Bass community among those that have the ability and fortitude to deal with bulky 100lb rigs.
Guitarists not so much. There are some, Sunns cop that Dick Dale surf thing very nicely (although I don't think Dick Dale ever used them
himself).
Several years ago I negotiated with the guitarist for "The Lost Patrol" (
http://www.thelostpatrol.com/) for my Sceptre, and my price and the logistics were too high to come to terms, but... he did get a Sceptre locally and AFAIK he still uses it. Whatever he uses, the haunting, tremolo laden, surfish tone he gets is pretty indicative of guitar tones Sundholm era Sunns do quite well IMO.