Update.
Amp on bench. Lid removed. 20 mins only spent looking around before bed.
Right channel now seems to be weak (compared to L) when warmed up now. So that's good or bad depending on how you look at it.
Tried different input sources(analog/digital), swapping L/R input (just in case).
Probing around as-is with no input or speakers connected.
It has a push button to sequence though 4 different speaker modes, A, B, A+B, OFF. Relays click on and off accordingly.
Voltage rails are +/- ~54Vdc around the preamp chips are as they should be, but here's the weird bit, drop down to 38Vdc when selecting A+B. The AC-in connector near the bridge rectifier reads from 84Vac down to 57Vac again, with A+B.
That seems to drop a lot of voltage only when in bridge mode and with no input.
Yet the power meter on the plug shows 30-40 watts between modes. No real consistency.