"801A Anode 500V, ~30ma , output DC-> mosfet gate, fixed bias, DC filaments."
Not a good attitude to develop first stage before the whole gain chain not clear.
Go back, from output, or at least from power tube grid swing requirement (depends of bias voltage!).
Grid swing peek voltage -> FET splitter: roughly define the B+ of splitter -> VAS stage swing: if it CCS or gyrator loaded, the tube anode voltage+half of swing + 30V spare voltage define the VAS B+ voltage. The VAS stage swing define the gain of VAS stage (assuming the 2V RMS maximum input voltage).
For example if the output tube's bias is 20V (40Vpp swing for A1), -apart from the splitter loss- the 801a able to solve the task, because the theoretical gain is 8 (practically can not reach it).
20V/8= 2.5V (peek), which corresponds 1.77V RMS input.
Not a good attitude to develop first stage before the whole gain chain not clear.
Go back, from output, or at least from power tube grid swing requirement (depends of bias voltage!).
Grid swing peek voltage -> FET splitter: roughly define the B+ of splitter -> VAS stage swing: if it CCS or gyrator loaded, the tube anode voltage+half of swing + 30V spare voltage define the VAS B+ voltage. The VAS stage swing define the gain of VAS stage (assuming the 2V RMS maximum input voltage).
For example if the output tube's bias is 20V (40Vpp swing for A1), -apart from the splitter loss- the 801a able to solve the task, because the theoretical gain is 8 (practically can not reach it).
20V/8= 2.5V (peek), which corresponds 1.77V RMS input.
"very best sounding" is subjective. I can not answer what "sounds best" to you. I'd go for something wide band, which is not always a given. A cascode with say a 10M90 top and DN2540 as lower device will be limited by the highish capacitance of the 2540 at the 10M90s Vgs. This works well: https://www.diyaudio.com/community/threads/ccs-keeps-failing.396350/post-7282507. IXTP3N100D2 might work even better as upper device. Put TVS diodes between gate and source of both devices if you are nervous. If you are good with your soldering this can be built point to point or use a little adapter pcb for the BSS159 to make it a little easier.
Choke: it depends. What lf -3db point are you looking for?