Since it's changing with moving the wires around, I'd say it's being picked up by a signal wire somewhere at either grid of V1. My first suspect would be the second gain stage, as it's running right close to the heater wiring. Possibly that other long wire, even though it's going to "not-V1."
I'd move those as far from the heater wiring as possible; also check that any other signal-carrying wires are well away from high correct AC sources. Looks like some of your PI wiring or parts may come close, too? Maybe even use shielded wire there also, if you must. Someone else may jump in with the "ground the grid" trick to possibly help isolate which stage it's coming from.
Personally, and I know it's a little late now, when I've rebuilt amps, I'm now a proponent of laying the heater wiring along the chassis like Ampeg & Marshall; as I build True PTP it gives me more freedom to fly wires high and I've had good luck with it.