Phew! that wiring job looks like it could use some improvement. You've got heater wires and power wires alongside signal wires etc and its like 'spaghetti junction' for coupling.
The layout also doesn't look optimal. I prefer to keep wiring as short as possible. You could also try running the wires right up against the chassis, and avoid running signal wires and power supply wires side by side. Generally preamp signal paths (which, starting with V1) are the most sensitive, should be as far away as possible from power supplies and heater supplies. Heater wires, although twisted together, look like they run alongside signal wires in several places - where necessary heaters and other power supply wires should cross signal wires at right angles, to minimise AC coupling into the signal path.
You could try shielding the preamp signal path from the input jack to grid of the first stage (ground the shield at the input jack end only, and you might need a grid stopper at pin 2 to stop any unwanted oscillation from the shielding).
Have you got the heater ground biased at a diferent voltage to the rest of the grounds? That may help.
How have you done the grounding? I would try and keep the pre-amp and output stage grounds separately grounded.