[GrimJosef]

I'm afraid that does happen. When the screws get lost, or the threads stripped, or the heads broken, rather than replace them some folks just get the soldering iron out.

Disconnecting the mains from the power amp, which is what the fault you describe would have done, shouldn't affect the audio circuitry at all. After all, we disconnect the mains every time we turn the kit off and that doesn't make it go quiet next time we turn it back on. When a fault inexplicably appears the first question I ask myself is "What have I just fiddled with ?". Since you'll have pulled the Jones connector out of the power amp I think I would start by wiggling the cable that goes into that and seeing whether the volume suddenly changes.

Cheers,

GJ