It would be much easier just to switch from 6P1 chinese tetrodes to 6P1P(-EV) Russian tubes. I'm running 6P1P-EVs in PP and getting 10W with no difficulty.
At 13W I get a slightly red fin on one plate (Have to turn the lights off to see it).
Sound is great.
I have a set of 6P1P-EV which have been swapped in place of the Chinese 6P1 with no audible or power difference. The need for improvement lies elsewhere in this amp, I think.
Not that it is bad like it is, but something still needs improvement sonically when my "afternoon project" EL84 SE (in pentode mode) amp sounds as good or better.
Gary