| Romy,
As you're planning to run a 6K load, you can run less current without the transfer curve suffering as much. So, another shot at some numbers.... rather than use a RC setup to drop the voltage, Hammond (not my favorite) make an open frame choke rated at 9H and 40ma with a 700 ohm resistance (154G). Using this would drop close to 30 volts at 40ma of current plus whatever you lose in the primary of the OPT. From 400 volts down possibly to 365 volts.
Using a 1.5K cathode resistor and a 370 volt supply, idle current would be ~39ma, plate dissipation at 12.25 watts and plate voltage at 309. Calculated distortion at 3.5-watts output is 4.6% 2nd harmonic and 0.2% at 3rd harmonic. The transfer curve looks decent and gets you very close to what you wanted.
Using the 45 with the 154G choke (LC for dropping voltage) and the 2K cathode resistor would yield the following: 32ma, 10-watts dissipation, 308 volts cathode-to-plate, 2.5-watts output with 5.3% 2nd harmonic and 0.2% 3rd harmonic. The transfer curve looks about the same as above.
In any case, you have a few options that all seem reasonable.
As for AC filaments on the 6A3.... not sure what your actual setup yields in measured output noise but would interesting to note which 6A3 tube you used and measured output noise. Using various 6B4G tubes it was unacceptable with AC. For output noise into an 8-ohm load I prefer a minimum of 80dB S/N referenced to 1-watt which equates to 283 microvolts of noise (maximum). You have a better chance at less noise from the 45 as it's only a single triode section where the latter 2A3 tubes are dual-sections strapped in parallel.
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