The EL84s in the ST35 operate with approx 14VDC on the cathodes, ao the maximum drive voltage swing would be +/- 14V. 6L6 s operate in cathode-bias mode, between 25 and 36VDC, and the ST35's VA/PI circuit can't develop that much of a "swing" without going into some SERIOUS nasty distortion.
The ST35's cathodyne phase splitter has a gain of slightly less than 1, at both cathode and plate, regardless of what kind of tube is used. And the 12AX7 VA stage doesn't develop enough gain to make the phase splitter go much beyond +/- 14V peak.
The PA-774 would be insuffucient for both filament current and B+ current to properly feed four 6L6s of any kind.
The ST35's PSU would work well in a mono amp with a pair of 6L6s or 6P3S's feeding a 9K PP OPT and 360 to 380VDC plate voltage, for about 24 Watts output. Use one filament winding for the 6L6s, and the other for the VA and LTPI stages.
/ed B