Everything says to bias the power tube plate current at the supply voltage divided by the output transformer impedance.

Who says that? That is so entirely completely wrong

You bias in function of how much dissipation you want at idle. So the quiescent current, times the voltage lost across the tube (difference between plate and cathode voltage) gives you how much power is being dissipated in the tubes at idle.

For true class A, you would be dissipating 100% of their max rating at idle (but doing so does not guarantee you're in Class A, but that's another subject).

Class A/B amp will usually be biased to 70% dissipation at idle, you can in theory get more output by biasing colder, but there are other factors at play.

So let's take your example. : You're dropping 20V across the cathode resistor, for the sake of argument let's say it's a 250ohm. 20V lost across 250ohms means 80mA (20/250=0.08)

Now, let's say you're reading 350V on the plates (loss across the OPT is negligible, measure B+ right on the plate to be sure). That means 330V across the tube (350-20 on the cathode resistor). 330V times 0.08A = 26.4W. Or 13.2W per tube assuming there are two.

Now let's say these are 6V6s, we pull their datasheet: http://tdsl.duncanamps.com/show.php?des=6v6
The PaMax (max power) is 14W. So if they are idling at 13.2W, that means 94% dissipation. Better tone might be obtainable at 100% (subjective), and more power might be obtainable at 70% (which you get by increase the cathode resistor, but doing so reduces voltage available to the tube which will limit output power). You also want a big fat capacitor bypassing that cathode resistor.

If what you want is max power, well the first thing to do is not do cathode biasing as you're wasting power over the cathode resistor. You'd do fixed bias instead so all that 350V is available to the tube and gain is maximized. Then you'd bias at around 70% (but you could do colder if you wanted).

The OPT's primary impedance as pretty much zero impact on biasing, because said impedance is only affecting AC, not DC. Meaning nothing gets through the OPT when there is no signal (which is where bias is being set). When you start getting signal, then THAT gets transferred through the OPT, and then the primary impedance will be a compromise between volume an linearity.

If you want more volume, you need a lower impedance, but you also need tubes that can flow more current, and a power supply that can provide that extra current.