Another check I recommend on the HH amps is the smaller filter caps on the pre amp board that smooth the +/-15vdc supply. They often have dry joints on them which you can see by gently wobbling the caps. However in this amps case I found that rather than having bad joints the blue paper just fell off. Another inexpensive replacement and recommended as a routine fix.
Another issue I have noticed with the HH amps is on the rotary switches used for bass-boost or valve sound. They do seem to suffer from noisy contacts. I replaced them on this amp as I found they click and popped when the shaft of the switch was moved slightly. Also hard to get shafts that are compatible with the size of the old HH silver knobs. However you can trim the knob shafts and make shims to fit them together snugly.
All fixed up now and sounding pretty nice especially with the valve sound is on (as it always should be set – in one amp menders opinion).