It is my understanding that Leo was not involved in the electrical design of these amps.
The power amp is reasonable, but avoid the multiplier if you need any power, get HV from a proper HV winding.Bias with a scope until you get rid of the crossover notch, we are talking very low current anyway, almost pure Class B
Of course you can build a single ended. It is just a triode stage, use two for push pull.Look up either "grounded grid" or "common grid" amplifier, both mean the same. The signal is fed into the cathode instead of grid. Music Man amps are maybe the best known examples of this. The various Peavey VTX series amps use it. Classic VTX, Heritage VTX, etc. And others.
The principals applied to triodes still apply to pentodes. Common grid is still common grid.Maybe I am lucky, to me the cathode drive seems intuitive. The tube works on the relationship between grid and cathode, not necessarily those elements and ground. Hold cathode at ground and wiggle the grid, you get signal at the plate. But hold the grid to ground and wiggle the cathode, the same thing happens.In a common 6L6 push pull output, I might ground the cathodes and have -50v on the grid. In a PV Heritage VTX, I might see +15 on the grid and +65 on the cathode. In both those cases the grid is biased -50 with respect to cathode. In practice, the amp examples I cited are running closer to class B than something like a fender twin Reverb would be.
Now, I'm very curious about the design process of such an output stage. Is there info anywhere about how to do it? I looked up "hybrid cascode push pull" but I didn't find anything close to this specific topology.