Albion Gulf Stream 30 - Possible Ground Hum Issue

My Albion Gulf Stream 30 head has developed a hum, which I think is a ground hum, but it's only audible on the clean channel (channel A.) So far I've tried swapping the valves around, trying different power cables, plugging in to different circuits, and cleaning the spade connector labelled GND on the PCB. None of this has worked.

I know it's a long shot, but does anyone have any experience with these amps and can offer a bit of advice, or suggestions on what I need to check?