I know exactly what is wrong. This model and the standard version will have the problem due to the fet's in the circuit going bad. I had one of both models and Sonic Frontiers repaired it. I think it cost about $140.00
Sonic Frontiers SFL-1 Siggie Hisses
abrupt onset of loud (OK, clearly audible at idle) hiss with degradation in sound quality from R channel of my Siggie.Swapped tubes, switched input, swapped input channels - no diff - swapped output to amp, switches sides to L. Any ideas about zeroing in on problem? I may have local (well regarded) guy repair it with schematic; already sent it back to SFI once for tune up in Canada, but this takes forever!