Sure. FLip off the lights, your ears work in the dark. Now you can tell how much of it is the lighting.
Does the volume control turn the hum down?
When you plug a guitar into the channel and turn the guitar volume to zero, does the hum remain?
Have you tried a different preamp tube in the end socket?
Is the heater supply grounded? Either a center tap grounded or the two 100 ohm resistors to ground?
Back the nuts off the input jacks, move the star washers, and retighten the nuts. Does this fresh chassis contact for the jacks help?