The first stage is to make sure the problem is in fact in the Moreplay. I assume that changing tubes means you swapped left/right the stock tubes? If so, then we know it's not the tubes It is easy and helpful to also swap the inputs left/right, and then swap the outputs left/right, to be sure it's not anything external.
The next steps would be to trace the signal by inserting a known constant signal voltage at the inputs and measuring the AC voltage level at the balance control and level control. PB usually suggests getting an app or website on your smartphone/tablet or whatever you have that is battery powered (to minimize grounding issues) using a headphone to RCA adapter cable. (He may have other/better ideas though!)