Interesting..... This is definitely taking these tubes to the next level of their performance capabilities. Definitely an "out of the box" approach
to amp design, something beyond my experiences.
I guess I'm much too used to the usual 365-380V B+, 35 mA cathode biasing and UL mode for these tubes.
I've gunned them up to around 25W, pentode mode, 400V B+ and fixed bias, and 6600 ohm OPTs, but lesser EL84s (like EIs and most J/J EL84s need not apply for the job)
The Russian EL84M / 6P14P-EVs are pretty tough, I've pushed them well beyond the normal PD ratings, up to 45 mA cathode curents, but at lower (360-400V), B+ levels than the RM-10's 700V
FWIW, has anyone tried 6P14P-ERs? I've heard good things about these, but haven't yet tried any.
I also wonder.... Since 6GK6s are basically 6BQ5s with a different pinout, how well would (or not) these stand up to 700V B+ and 350V screens... Then there are the lower cost 10GK6, 16GK6 and 29GK6s.
Roger has also done some interesting designs with our favorites, 6EM7 and 13EM7:
http://www.ramlabs-musicreference.com/EM7.html
/ed B