Blair ...
You have some serious grounding issues.
The power supply has grounds connected right into the audio path - for example, the capacitor for the driver tube is grounded right to the volume control, the main high voltage ground kind of connects to the input stage. You seriously need to run a bus wire, like a piece of # 12 house wire, insulated from ground. I'd run it pretty much down the center and make it about as long as the distance as end to end of the tube sockets.
On one end ... connect the center tap of the plate transformer and the negative side of all of the filter capacitors, very close together on the bus. Keep any audio grounds down at the other end. Use an insulated phono jack, and shielded wire to connect to the volume control. Get rid of that speaker ground on the input jack - that helps create a ground loop.
Basically now, you have the negative side of the power supply connected to the input of the amplifier, and the music is fighting the hum.
Take a look at some of the pictures on my website, and look at the grounding schemes. Remember - keep power supply grounds together, and audio grounds together and don't mix them closely.
http://tmamps.com
... tom
