[bikerhifinut]

Another thought, what happens when you short circuit the grid to ground? on the grid side of that coupling cap and then on the source side? somethings not sitting right and it could be something in that area? i usually do initial testing with a shorting plug in the input socket to shunt any extraneous induced hum etc to earth. If you have simply connected the inner of the coax up and left the socket open circuit you're pretty much guaranteed to get a bit of hum.
20mV p-p quiescent hum seems rather a lot to me too, I got my amp well below 1mV.
I also see there's no grid stopper resistor, I'd put at least a 1k there as a standard safeguard against HF oscillation. My memory may be playing tricks here but I seem to remember being told that HF oscillation can result in an audible hum as a side effect? I need to look it up in the textbook. Anyway using a grid stopper means you can safely short the grid to ground as would happen with a normal volume pot wiper connected to the input and turned to minimum.
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