The heaters are a separate circuit from the audio. If you want to lift the C30 preamp circuit and build it elsewhere, you are not obligated to use th PV heater circuit too. All you need to do it get the 6 or 12v across each heater. DC will have the potential to be quieter, but plenty of great sounding and quiet preamps have AC heaters too. Good practice makes good results regardless of the approach.
PV runs the heaters on DC and wires the 3 tubes in series across -36VDC. You could do that, but likely your power tranny doesn't have a winding for 36volts. SO just run the heaters on 6 or 12. You wanna make it DC? fine.