Ok, so I came across an old episode of SNL on E! a little while ago and the guest band was playing. They were really odd so this shouldn’t be so tough. Each member had a Euphonium/Baritone or some variation of it. It didn’t look quite like one but they were all low Brass instruments (And I think at least some were 4-valved). Well, there were maybe six of them or so. One was tearing up on lead, while the rest were all doing a melody or something or other. I can’t remember the song now, but I suppose it was kind of funky or jazzy.
Remember, this was on E! so it was probably in the 70’s or early 80’s, maximum. It was purely low Brass, there was definitely no saxes, drums, guitars, trumpets, keyboards at all. Perhaps 'bones, but probably not.
The SNL is the clue: It was Howard Johnson’s Gravity. Johnson is a top jazz tuba player and was in the SNL band in the early years. Gravity performed in one SNL episode.