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.