SSE Noise Problem - Help?

Hi
I have an SSE, about 6 years old. I use the SSE for the treble end of a bi-amped system. One channel is noisy (loudly hissing, sometimes pinging) the other almost silent.

Swapped output tubes - no help
Replaced output tubes - no help (except no pinging as yet)
Replaced input tube - no help
Recapped entire board - no help
Re-made all terminal connections - no help

No matter what I've done the same channel has a problem, the other channel doesn't.

Anybody have any other ideas? Could the 10M45S be the problem?

Thanks,
Charlie