I'm a studio technician working on the amp noted above.  
The amp has a constant 120Hz hum at about 200nW or -58dBV.  The amp also exhibits pretty significant distortion at about 330mW on each channel, significant being visible disruption of the sine wave (not clipping) on the negative half of the waveform.  THD jumps from 1.5% at 200mW to around 10% at 360mW.
Both channels affected.
Frequency response is also poor, -3dB at 20Hz, and 15kHz with steady decline from 1kHz to 15kHz.

I added 200uF of capacitance to the high voltage power supply and the hum dropped 13dB, but no change in power output.

+320VDC going into output transformers
9VDC on cathode of output devices, replaced the capacitor here just in case, no change.

This is definitely in the output stage, signal on the grid is clean far beyond where output distorts and signal on the grid doesn't have the distortion of the output.

What am I missing?