[EdHolland]

Thanks. Very interesting.

Is the ~460 mA the current in your 40 Ohm resistor? Filament current for the 6-series tubes should be around 0.3 A if I have the correct data.

Other thoughts: The 2M, 47 K resistors and the volume control? I think I read that you checked these, and mentioned that grounding the input to the 6SQ7 doesn't kill the noise, but something could be intermittent.

Clean the socket, and check contacts? This should reveal if you wiggle the valve as the problem develops during warm-up.

Best of luck!
Ed