The heaters in all the valves come on and glow brightly, so I know the heater supply is ok. I've checked over the PSU and the circuit seems fine and the voltages are what you'd expect when there's no load being drawn. Both channels are dead so I think you're probably right that it's something common to both channels which in my mind suggests either a fault in the B+ supply at the amplifier end or a faulty valve. I've got a pair of old mains powered computer speakers that I assume could be used as a test amp. I take it I'd just rig them up so I could connect them to the output from each valve in the amplifier section and see if there's a signal when the power is applied?