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For the schematic vs the layout, you are absolutely correct. The extra 1M resistors on the schematic are for the pop suppression modification. We include those parts in the kit as a courtesy. Its a nice modification to make, but its not in the book and the layout is straight from the book.
For the OT, I usually do GREEN and YELLOW. It depends on the number of speakers you are using and which tubes you want to switch between. Since I use both the 10” and the 8” at 8Ω each, my impedance is 4Ω.
So GREEN gives me the 6V6 at 4Ω and YELLOW gives me the 6L6 and EL34 at 4Ω.
If you had 1 speaker at 8Ω, you would want WHITE for the 6L6 at 8Ω and YELLOW for the 6V6 at 8Ω.
If you wanted to stay with the 6L6 tubes but wanted to switch between two and one speaker, then you would need to switch between 4Ω and 8Ω impedance so you would do WHITE and YELLOW.
When you are reading the chart on the 125ESE, you have to look at the PRIMARY impedance to know which lead to use. There is a paragraph in the book that *kinda explains about the mismatch being ok here. You are looking at the 5000Ω primary if I recall correctly.