Yes, it looks like I have been arguing the theoretical mu of the stage and you have been presenting the actual gain of it, heavily-loaded.
I think one of the reasons that the McIntosh amps used such a heavy load was that the screen voltage was so high. You have to have a steep load line to hit at or above the knee of the Vg=0 curve. (The other reason was probably to have big Pout numbers.)
That was a big complaint I have about the Plitron UC transformer, the load is too light. I had to find a way to lower the screen voltage with mosfets so that I wasn't causing damage to the screen grids when the amp is driven to clipping, since the load line would drive into saturation under the Vg=0 knee.
Otherwise, the Plitron transformers are very nice. Very low winding resistances and unbelievable low-end.
I have never had the chance to hear a Mac amp. I don't know any other tube nuts IRL. 🙁
I do like the sound of my amp, though.
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From what I learned over the years wrt to load impedance (Broskie and others) is that it does not make a difference if an output stage is classical push pull, split-load or CF.
Your 6k4 transformers seem to be a good load (apart from the screen grid thing).
The Lockhart design also had 6k primary load impedance.
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