Testing the BSTRD - Success... Not entirely true at first; I had used a 330 ohm resistor in the feedback loop instead of 330k. The result? No sound. At all. Not even noise. What a bummer...
Well, replacing the resistors made all the difference. It was a real joy to see the 1 kHz sine show up on the scope, nice and clean.
I hooked it up to a power amp and a single speaker (I tested one channel at a time), and all I can say is that this thing sounds sweeeeet. All the way from the meaty bass to the tingly treble. :)
Note that the PSU needs two transformers, because of the 78V part using a voltage quadrupler. Two tube filaments drawing 300 mA each, is 600 mA in total. The regulator and the heat sink get really, really hot. I should probably find a way to get more cooling, it will most likely mean moving the regulator off the PSU board. UPDATE: The excessive heat was because I used way to high AC input on the 6.3V side. Replacing the 15VAC power with 9VAC made it all better.
I measured the regulated outputs for AC, and my multimeter shows 0.00 mV on both the 78V and the 6.3V. I am quite happy with that result. :)