The light is on the front, but the circuit is in back on the 260C board.
Are both polarity main power supplies present, roughly 42v each? Scope the output, what does the distortion look like? Missing half waveform? CLipping severely? Other?
Your main IC on the 260C board is a TL074. There are several of those on the preamp board (mixer, front panel). Swap the TL074 in your power amp for one of the ones up front. Any change?