For some reason the hum completely disappeared.
You had a ground loop. They can seem quite tricky and mysterious. Probably what was happening is the way it was grounded had something to do with the gain stages in the preamp- that they were obtaining their grounds the same way the filament supply was, or that they both get their grounds through the same circuit. That way the current of one circuit becomes conflated with the current(s) of another- allowing for that current to become a noise vector somewhere in the circuit.
At any rate I'm glad you got it sorted!