100 hz hum tube
If I understand right, the FET at V2a input is used for muting during switchover only. The amp should also show basic operation with the FET replaced by a short. It shouldn't be able to cause the hum interference (or at least, I don't see how).
2N5462 has a considerable higher Ron than J176, but this shouldn't matter much, I guess this is true for all J176 devices in the circuit. J176, by the way is in production with Fairchild and Philips/NXP and on stock at catalog distributors, as I see, but I don't think that it's necessary to replace a 2N5462.
You should have noticed from my previos post, that I'm not confident regarding filament AC causing the hum, but I would perform the DC test to check further (if it's possible with acceptable effort.
Otherwise, I'm slowly running short of explanations. I also mentioned possible RF oscillations, but they seem very unlikely for the present circuit. I don't know what's your oscilloscopes bandwith, but as I said, oscillations should be noticable at the affected tubes (V2a, V3b, V3a anodes or V3b cathode).