Op Amp for Heater Supply?
Three comments:

1) You'll want to regulate the voltage down to a stable voltage, say 5V, using a high-line-rejection regulator. A simple linear regulator with a couple of capacitors can provide > 80 dB rejection here.

2) You're presumably using a tube pre-amp to get a sound that has significant tube (saturation?) distortion instead of possible transistor (zero-crossing?) distortion. Why would you then introduce transistor distortion from an opamp on the output signal? Isn't that kind of counter-productive?

3) All tube characteristics (linear and non-linear) can be modeled efficiently in a low-cost DSP, to within limits of measurability (which are more sensitive than human ears.) So there's probably a much cheaper and easier way to get the sound you want, with less energy waste and smaller space.