"The schematic above, did you build that in Spice? again impressive how quickly you turned that around."
I made many thousand simulations (very much with tubes) in the past (almost 20) years, so building new (if the appropriate models presents) with available blocks is relatively easy.
In most cases the "more linear" operation (which goes together lower distortion) mode coincides the "better tone" feeling, so if you don't chase more and more power, higher loaded power tube is better solution.
I use (more than ten years) 5k OPT loaded (CCS loaded D3a, FET SF direct driving fixed bias power tube) 300B SE amplifier which is have 1.1 .. 1.2% THD near at A1 border (6W).
Due to the beefy driving at 10W the distortion only 1.5%.
IMO the driving of power tube is one of the most critical designing mission.
If it possible, I use direct driving (CF or SF) solution: driver low impedance/driving capacity is one of the main design parameter.
I made many thousand simulations (very much with tubes) in the past (almost 20) years, so building new (if the appropriate models presents) with available blocks is relatively easy.
In most cases the "more linear" operation (which goes together lower distortion) mode coincides the "better tone" feeling, so if you don't chase more and more power, higher loaded power tube is better solution.
I use (more than ten years) 5k OPT loaded (CCS loaded D3a, FET SF direct driving fixed bias power tube) 300B SE amplifier which is have 1.1 .. 1.2% THD near at A1 border (6W).
Due to the beefy driving at 10W the distortion only 1.5%.
IMO the driving of power tube is one of the most critical designing mission.
If it possible, I use direct driving (CF or SF) solution: driver low impedance/driving capacity is one of the main design parameter.
A couple of years ago this was my idea of a 300B SET output stage driven by interstage xfmr, but not only an idea because the second graph shows the practical frequency response. The next immediate step was adding an input xfmr (1:2, 1:4) with exactly same response, all xfmrs made by me 
@Matthew78 - So, keep the faith, it's doable
@Matthew78 - So, keep the faith, it's doable