The Hum was caused (of course) by faulty capacitors. In this case it was the filter capacitor on the negative bias voltage was a complete open circuit, causing my bias voltage to be around 40V AC RMS. This was causing the hum. In the end I did a complete recap of this amplifier, installed two 10″ alnico speakers pulled from a Garnet, and installed some NOS potentiometer knobs. I even re-did the grill cloth with a nice grey fabric. I think it turned out great.
This amp was full of these cheap old oil capacitors, and it looked like it had never been serviced. I should have done a recap right from the start – it would have saved me time, and a Part 2 wouldn’t have been necessary.