The circuit of those amps uses the tertiary widing in the OPTx for cathode feedback. Cascaded diff amps, a 6BL7 to a 7236 was the last combo I tried. It sounded great. The OPTx was definately the limiting factor. I am designing a replacement on a smaller core with a better interleaving schedule. I am aiming for the Quad II 9 3/8:1 ratio. It is taking a bit of iterating, that's for sure.

Remember, the flash caps are a budget and space expediency, I don't think they are designed for continious charge storage. They seem to survive it adequately( and I can put together a few hundred uF from a parallel connection ). If I didn't already have most of a chassis ready to go, I'd use the motor run caps I have in a box in a second.
regards,
Douglas