Too much gain and possible lead dress issues (can't see that part). Looks like too much low end through the preamp for a cascade high gain circuit also, though that's not what's causing the oscillation. Possible microphonic preamp tube issue but I think the circuit would oscillate anyway. To effectively employ this much gain it's usually necessary to use VERY careful layout and bleed some top end out of the signal chain to keep the amp stable. I also don't like the way the inputs are rewired. It removes nearly all options normally offered by the four input arrangement, so what's the point in having four inputs after that? I don't like this mod. IMHO it's clunky and poorly considered. I promise you that Rob Robinette has never built this amp with stable results. I'm sorry I don't know what to say beyond this right now. The circuit might be redesigned to work, but since you're new to this I hesitate to try. Taking on a really high gain circuit is tricky enough for experienced builder/designers. It's A LOT more than just the schematic. It involves component layout, grounding, voicing and phase and noise considerations.
Sorry to be such a bummer. There's just so much to fix about that circuit and there's not much to the circuit. It would be better removed.