Ok, here's the gist of my frequency indicator. I'm not saying an audio amp should necessarily use the same approach, but it could work.
I ended up using only 4 inverter stages for the PSU, which gave me about 50V from 12V input, but for audio I guess all six would be preferable (it's a hex inverter chip) especially if its to run off 9V, or use Rick's SMPS of course. This circuit runs at about 300kHz, so I guess it won't cause audible problems.


This shows how all the tubes are biased to about -2V. For audio, to prevent feedback between stages via the heaters, I think you would want to split each grid leak into 2 parts and decouple the junction of the two resistors.

In the frequency indicator I just had an opamp input stage which feeds eight low Q bandpass filters
http://sound.westhost.com/project63.htm each tuned to frequencies from 50Hz to 10kHz. Each filter then drives a tube.