A late update:

Seems like it is the AC sensor switch that's at fault - I've yet to absolutely confirm it, but I'm pretty sure that's what's wrong.

Guess I've got to decide whether to send it back to support, or get my Dad to fix it - anyone know the specs of the switch? (I assume it's built into the AC power socket?)
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Empeg Mk2a 128G with amber lit buttons kit - #30102490

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