Okay I have read through some theory again. The power supply to the transformer is in fact a tunable parallel rlc circuit (resistor, inductor and capacitor). the capacitor we know about. The inductor is the coil in the transformer and there is a resistor in the neon bulb.

The combination of these in an ac circuit will have a complex performance but they can be selected to produce a bandwidth of frequencies. If the frequency to be desired is set at say 16hz that would not have been achievable in that era of amplification and would not be reproduced suitably loud enough. Thereby reducing the noisy 60hz.

A crude but reasonably effective solution, where HiFi isn't required.

Using: Dell 3268 i3, 8 GB ram, windows 10. Biab version 2017 standard package, no extras.