300B PSE Loss of Volume, Bass, and distortion

[Richard]

Very thorough David and let's hope it has fixed it.

If not,

"When I rebuilt these, I reinstated the feedback loop, which a previous owner had cut out. Having experimented with both settings, I ended up leaving the feedback off, and that is how the amps were when the fault first appeared."

I had problems when rebuilding my 300BPP with switchable fb from a poor build so I removed it as it sounded better without fb anyway.

It would be easy to hardwire the junction of R5/C3 to signal ground to eliminate a bad switch or connection in what is a sensitive part of the circuit which would affect volume/cause noise if there is a problem.