Tranny failure

[Mike Watterson]

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.

It was a confusing statement: "but stick a sine wave in and the OP power goes down"
Down from what? How was it measured without sticking a sinewave in? I made the probably unwarranted assumption he really meant the theoretical power dropped, due to PSU voltage dropping.
Many stereo amps could give more power on one channel alone, power dropping if both were loaded.

Yes, absolutely all you say is true.