The owner cannot detect a difference. Other than different gain settings are obvious, how to set up and test? . I've not put a scope on there yet, but what set of parameters to confirm there is a difference, compressor or no compressor, max or min of contour settings, what sort of level and frequency of presumably over-driven input? Test not requiring guitar input and ear-drums, as then it becomes subjective/musical ears required. The valve actively tests fine.
Incidently seriously crap "engineering" on the speaker outlet board, especially as it is PbF. 2 legged 1/4 inch socket, no extra dummy legs for mechanical strength, so 2 simple active PbF joints supposed to hold the pcb and 2 thick unconstrained supply wires , laughable.