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12AU7. Can you explain to me how different value plate resistors would improve or degrade the operation of the circuit? I'm just curious, really. The only thing I can think of is higher value would improve PSRR, true?
The plate resistor or anode resistor is allow voltage swing and customize gain. At a minimum you should pick 2*Rp where Rp is about 7700 for the 12AU7 which is a good point to maximize gain and voltage swing. 15K is a good start, though I would look at shifting the operating point to 80V on the plate. Broskie had it at 1/2 B+. For you, that would mean a grid of about -2.2V, or a bias of 5V. You don't have to do that, but playing with it may be fine. You'll probably want some deterministic tests to go with it. Lower noise, etc., through measuring equipment.
The divider circuit does 2 things, when from tube output. Bias *and* negative feedback to reduce gain and linearize the local stage. Biasing from that point is advantageous, as opposed to B+. Do not bias from B+. You will inject PSU noise directly into the grid. Bad, bad, bad. If you do use B+, you will need a transistor in there, IMO, for more isolation.
For a hybrid you want to reduce gain anyway, feedback helps a bunch there. Local or global, up to you. Even the SSMH had feedback, though non-tube people and newbies would never know it.
Anyhow. I would play with this after you get the circuit operational and functioning. Since I see you started already, on LED biasing, does it work? Do you get sound, are the DC operating points where you want? What ever you do, bias from the 106V. I would not worry too much about plate voltage. It doesn't need to be precise, just close enough. Tubes and load lines are not 100% accurate anyhow. For 80V plate, -2.2V grid is a bit low, but that is about where the SOHA 2 ran (48V plate, 2V cathode @ 1mA, just about). I don't think you'll clip, but am not sure. Any how, for a quick test, you can just use a 5V zener. Yes, it's noiser, but for testing it's quick and dirty.
On a side note, I would probably feed the voltage through a 300 ohm grid stopper resistor as well. Minor tweaks that you can do after you get up and going. The grid stopper is for stability, not really needed, but I would do it anyway based on common practices.