Cheers Martin. Had the amp to the tech and as is always the case it worked impeccibly for him! Checked valves and gave them all a clean bill. Asked him about caps and he says they all look fine (honestly dont know if/how he tested them. As such ill pop in back for sale and will give it a few hours use before any sale to check - obviously if the hiss returns ill get it sorted immediately.
When i say his tho i do mean a very faint noise that wasnt there before, certainly not full on gain-cranked type hiss or crackly pops. As before any trial welcome and would even consider hiring a small room at the local rehearsal space for a full on cranked test drive for any prospective buyer