Do a "search" or simply broswse the forum for posts (many by me) on the reasons for the "old" Sunn sound.
Sunn made three basic amps:
-- 2 KT88/6550 & huge transformers
-- 2 EL-34 and large transformers
-- 4 6550 (I have no experience w/these beasts.)
To these they added a
-- guitar preamp, one with, one without effects
-- various bass preamps
-- PA (i.e. multi channel) preamp
Under these they put a range of speaker cabs. Others can be more precise on this.
Sunns always had a vey flat response, much less of a notch in the sound than Fender. They were also cleaner and louder than most competition. This, plus the early use of big (15') JBL speakers and ported enclosures, made them immediately popular w/bassists.
Some of us kids saved all our coins to buy a Sunn because we didn't like Fenders. Most of the big name guitarists who tried Sunns dropped them because they were too clean. (Remember, fuzz/overdrives were very rough and primative at this point - think "Satisfaction." The creamy, Claptonesque "woman tone" came from midrangey Marshalls and tons of 25watt speakers. Sunns did none of this well. Sunn fought back by adding a mid boost to their amps and producing a 4x12 model, but it was too late.
I leave it to others to describe the new, now extinct "Fender" Sunns. Most people who have tried 'em, love em. And, once again, Sunns are a great deal for players, vice collectors, as they are cheap.
I was fortunate enough to buy a mint '69 Secptre with the 4x12 alnico cab. I play a Lone Star 'strat and a Heritage 535 and it sounds great with either.
I am running JJ KT88's (but the original RCA 6550's are still perfect).
I play around with the preamp tube, currently running a JJ 12AT7.
This beast doesn't cut through the mix, it bulldozes it. The sound is huge and everyone who hears the rig loves it. I have a Visual Sounds RT 66 and a TSX-9 in front of the amp, but, in fact, rarely use them. Certainly the compressor is never needed at stage volume.
Sooo, bottom line. While the Solarus (w/EL-34's and 2x12) and the Sceptre (w/4x12) are the most "modern" of the old Sunn line, I'd kill for any of 'em.
The JBL 15's have a great sound. I used to get fantastic sounds with a 335 and a 15"&horn cab. If the horns add too much sizzle for your taste, get a tech to pad the horn more, or bypass it alltogether.
Sorry for the long post, but this forum has been a bid dead since Fender pulled the plug on the new Sunns.
So lift a glass to Conrad, play loud, and let the Sunn shine!