hum-loop-like problem

Laney VC30 valve amp, works fine in its own accord. But owner wants to set it up with another amp , another make and solid state if relevant, for a pseudo stereo set up using the A and B outputs of a pedal effects unit. Produces mains hum on switching from standby to on. Making one of the feeds isolated and relying on the grounds from each amp makes no difference, still intrusive hum. Bridging stout wire from mains earth to the VC30 input socket makes no difference . Pulling V1 from the amp still induces hum in the other amp. Had to return the amp for owner to do a gig as normal mono set up. I did not try an isolation transformer and I did not get inside the works, just testing the valves. I'm assuming internal shared ground path for HT return current , anyone any thoughts for the next time if I'm faced with this. Only a year old so maybe PbF problem at the input perhaps