Things wot are goin' to help diagnosis - replace C48 - take out all of the valves - take the voltage on say, V4 anode (plate, if you're American), and then the grid and tell us.
Check the voltage on the two rectifiers D7 to D10 and D12 to D15 - the transformer side will be AC and the other DC - take the voltages ref to ground.
Check any components in the recifier chains for discolouring, including the rectifiers.
Bit uncomfortable about 'old but working' valves, and next we can check bias voltages, but we need a running chassis to check the voltages, otherwise we start checking basic resistance.
Also uncomfortable about you 'then hooking up the cab' - valves and o/p trannies don't tolerate no load......might have to check the o/p tranny.
Back at 1400.