So the bassist in my band has a pre-1975 Ampeg SVT amp, nothing is necessarily wrong with it, and everything is fully functional but it seems to kinda lack output, it definitely doesn't feel like it's pushing the full 300 watts. My 120W Ampeg VT-22 guitar amp is a lot louder than the SVT, and I'm not just talking by a hair here either.
The obvious culprit here would be the tubes, neither of us have money for new tubes but I do happen to have a lot of the major ones in his amp leftover from various retubings that I've done to my Ampegs. I do realize that ultimately, it's going to come down to us sitting there and trying every tube combination, but what would you say would be a good starting point? Are their any particular tubes that you've found to make more of a difference?
..I was watching my death.
First, what kind of cab setup is he using? It could be that he's just not moving enough air.

Second, are you saying you're planning on retubing his amp with old leftover tubes, or are you saying that's what he currently has in there?
Composite Aficionado

Spector and Markbass
He's using the original 810.

I have a some Magnavox NOS tubes, all functional.
Looks like my best bet is to just giver er the old plug 'n' pull and test out every tube in every socket.
..I was watching my death.