I had one of the solid state Coliseum 300 heads in the late 80s; the ones that came in wood cabinets. In retrospect I think that product may have been designed after Sunn’s sun had begun to set. It was notable only for how heavy it was.
These amps look interesting.
If you are referring to the original, “silver face” versions…
…those are far from the “downhill” era of the original run of the brand(the Concert line was the lesser wattage, downmarket stuff). They were seen, in the early to mid-70’s, in all sorts of pro backlines, even the guitar amps in spite of being solid state(Clapton, Beck, and Rundgren used them at various times). The series was in development while company founder Norm Sundholm still owned it, but he had a falling out with his brother/partner, and they sold out to a large conglomerate, Hartzell, who moved ahead with the models. In fact, while Sundholm did dabble with a few transistor combo amps earlier on, the new guys ran with it, phasing out all the tube gear, save for the token Model T, which was an all-purpose head.
As the decade moved on, with new competitors like Peavey to deal with, Hartzell did redesign and cut manufacturing costs, such that the newer versions looked like this…
Plus the introduction of the Alpha and Beta lines. While those do have their fans, overall this is the era in which it’s perceived that Sunn went downhill, and sure enough the company sputtered to an end by the early 80’s. Fender bought the brand name, issued some PA gear under it, then mothballed the imprint until the late 90’s when they briefly revived it with a series of not quite reissue amps that utilized 6L6 power tubes instead of the signature KT-88 or 6550 of the 60’s era heads. The Fender amps, tube and solid state, weren’t bad, just different animals.