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Very, very nice!
All the above is very good advice.
I would add that the heater voltage should be measured at the PX4 pins with a DVM to make sure it's as near to 4v as possible.
PX4s do not like being over-run, it reduces their life quite drastically.
Measure the hum at the speaker terminals with a DVM, what is it?
Converting to DC heaters will reduce the hum somewhat.
One of the PX4s is clearly near the end of it's life, as stated previously, when you feed your sinewave in, measure the (AC) voltage on the anodes will show how much.
Lovely kit, have fun!