> It has a low and high power switch which I assume turns one of the EL34's off. If that is true...
It isn't true.
> How could you run a push pull amp on one tube? Wouldn't you only get half the sine wave?
If tube amps ran perfect Class B, cut-off at idle, then it would be true.
No tube amp runs that way. Tubes have a soft cut-off. If you could push them all the way to cut-off, there would be severe distortion for small signals as they crawled their way out of cut-off. (This does happen in badly biased transistor amps.)
If the tubes ran Class A, removal of one tube would reduce the output power (and annoy the transformer with DC current) but within the new power capacity it would run single-ended producing both sides of the wave.
Most tube amps run well up into Rich Class AB, as much A as B, so they will sound fine one-legged, just a bit weak.
And possibly VERY bass-less, because a push-pull transformer does NOT like unbalanced DC in the windings.
> Very interesting design driving the cathodes with the signal instead of the grids.
It is a transistor amp with tubes doing grunt-work, NOT a proper tube amp. Below clipping the distortion is entirely transistor-like. Any tube-curvature is reduced about 500 times by transistor collector action. They take enough NFB around it so it is dead-clean up to the clip-point. Very 1980.