11-15-07: Springbok10I see.
What I mean is that you cant use a 6 series with a 12 series in line - so if a 12AU7 is on the (LF) outside, another 12 series tube must be used on the HF (outside), (according to George) defeating the purpose of using the higher gain tube.
If it were me, I'd try it anyway despite what George says.
Otherwise, I'd investigate the Marchand tubed crossover.