[Diabolical Artificer]

Personally I'd use two 6SN7's, one for the IP for both L&R channel, one for the PS. Phase splitter run from CCS and - 80v negative rail. An EF37A & 6SL7 have far too much gain unless your intended to amplify a TT cartridge.

An 6SL7 is a high gain low current valve, you want the opposite to drive an OP stage cleanly. You want lots of grunt, IE high current to drive the OP valves inter electrode capacitance. An excellent choice is an ECC82 or better a 12BH7 or a 6BL7, though a 6SN7 does the job.

Any valve amplifier depends on the OPT, get the best you can and the biggest, EG for a 20w amp get a 100w jobbie. I've attached a schematic for an 807 amp I built. Recently lent it to a mate who paired it with his sensitive speakers, he said it's as good if not better than any valve amp he's listened to, including Quad, McIntosh etc. I built it from scratch, pretty easy to do if you have time and patience.

There's a few mistakes on the schematics, they're for guidance only.

Andy.