I've given the amp a good clean up inside and had a look around to see if anything obvious was amiss. Now as I said I'm no amp tech but I couldn't see any obvious issues, burnt terminals or boards, swollen components or anything like that, and I know you can't tell much like that but it worked when it went into storage years ago and has been dry and warm indoors, the trunk it was in is beside the fireplace with a woodburner in, so I've dug out my old cabs with the celestion speakers in and given it a low volume test at home. The speakers are old Rola Celestion G15-150 8ohm, this is a guitar speaker but I used to use a pair of them for the bottom end of my bi-amped rig, basically they were used as subwoofers and that rig sounded amazing and was seriously loud, size and weight stopped me using it. I know they weren't handling full range as I'm asking of them now but surely the bottom end is the end that would've killed them if bass was going to?
Anyway I'm going to take it all to rehearsal this week and try it at volume. The 15's are both in separate cabs and I can't remember if I wired the sockets to join them in parallel or series so the easiest way I'm guessing is to plug them both into the amp separately, each into one of the 8ohm outs on the back, I assume I can plug two cabs in at once. I might only need one but I'll take them both in case. (EDIT) I was having a dumb moment, they must be in parallel to give 4ohm, there's no reason I'd have wanted 16ohm, for a moment of stupidness I though 8 or 4 but obviously it would be 16 or 4.
At home I tryed it with a P bass with flats and one of the Celestions and it really sounded quiet nice, a valve pre would obviously improve it but it was warm and vintage with plenty of thump, I was a bit surprised actually as I though it might be a bit quiet and middy but no. I banged through 'Get Ready' by The Temptations and it sounded bang on, until Mrs. Maude reprimanded me 😁
If it behaves at volume and sounds nice then I'll get it checked over properly and replace anything it needs.