I would be inclined to use a 400-0-400 to power the whole works, and drop the B+ for the preamp with a mosfet regulator if needed. Or just RC filtering. It would be possible to use a separate transformer to power the SN7’s, but on something this simple, why? You might do it if you were using direct coupled drive to the output tube, and needed the whole front end to come up and stabilize before throwing the switch to the final stage B+. A SET running in A1 isn’t going to need that. If you had something like a MC phono pre, you might want a separate supply to keep noise and crosstalk down. But nothing like this is going on here.
The schematic you show uses cathode bias, where a separate voltage isn’t needed. I suppose you intend to dispose of the cathode resistor and feed in a negative Vg1?