The 12ax7 is used for input/gain and wonder if the 6h30 is somehow used as a driver for the SS amp section,
you are spot on, the X7 has a max gain of around 100 , runs at low current, (us around 1mA) and therefore has very poor driving ability, the 6H30 has a theoretical gain of around 16 and runs at a much higher current (us 15+mA), conferring excellent driving ability. It may even be configured in a cathode follower configuration, which sacrifices gain (only 0.9) for even greater grunt.. The max power dissipation of the 6H30 and 6N6P is 8watts both plates driven for both tubes, so I wouldn‘t be concerned about catastrophic failure if you made a substitution. The plate curves are somewhat different though, and the biasing would not be optimal, although the outcome might sound fine (you might still be in a fairly linear area of the plate curves).
A schematic would be great, and I would really pressure the manufacturer to get one.