Don't forget PSE 4P1L. There's a good thread on using this in filament bias with Rod Coleman regs. It's what I use myself. Tubes are very cheap and HT is around 300v. Very similar to using a 45 (= one 4P1L) or a 2a3 (= two 4P1L). The usual 3.5K OPTs all work a treat. Plenty of threads on 300b OPTs. Check out NP acoustics in Vietnam. Good prices on amorphous cores, and a very nice guy who loves talking about his work and making audio friends on Facebook.