Behold, Jon Yaeger signalled from keyed 4-1000A filament:

Further, what are the sonic implications of using an (actively)
regulated power supply with a tube or MOSFET pass element?

I have used MOSFET and bipolar elements in my B++ regulator designs.
Personally, I prefer the bipolar, but the impedance of either should give
you less than 1 ohm down to the millihertz range, because any regulator is
in effect, an amplified capacitor as well.

Either will do justice to any class of amp.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca