It might be interesting to set the two different pairs up with
separate drive circuitry; then a "balance" control could be
added upstream to adjust the drive proportion for each pair.
I would think you could use one phase inverter. The
difference between the el84's and 6v6's is going to be that
you will want to put more resistance between the phase
inverter section and the grids of the el84's than the 6v6's.
Well - I thought I had a trick here, but you guys are close so
I'll give it away. I've got an amp that runs off a pair of each,
and I use the pwr amp MV circuit where you replace the
grid leaks of the tubes with a dual pot. Each set of tubes has
it's own dual pot, so you end up with a 6V6MV and a
EL84MV. Very cool way to give you any combination you
want.