Tranny failure

[Murphyv310]

but stick a sine wave in and the OP power goes down

Unless it's a regulated PSU, that's normal.

What??? That's an open statement Michael. If the OP is checking the amp in the correct way, (power meter, scope, rated sine wave at the frequencies stated in the manual, dummy load etc) the Power should be as per the specification laid down by the manufacturer.
If the power sags below the rated sine wave output then there is a fault no arguments.
Actually testing an amp with music doesn't give a proper RMS figure, it will more likely be a PMPO figure so any accurate testing can only be done with the above equipment and a sine wave.

I have found many transistorised amps to be down from spec, usually caused by small changes in resistor values, transistors that exhibit small leaks, power supply caps with poor ESR and other maladies, transistor amps are rarely better than their valve cousins, both suffer from cap and resistor troubles, at least on a valve amp the replacement of valves takes seconds rather than a full dismantling.