Hi Kenneth and welcome to the forum!
We get this question a lot, so I’m going to try and give a fuller response than normal. The caveat here is that this is my interpretation of Steve’s design decisions, so I stand by being corrected by someone from the shop.
Tubes operate in quite a wide operating range. Bias is adjusted to squeeze every last bit of power out of the tubes at a reasonable THD level. The Power Station assumes a matched pair in the “medium” range of your supplies’ anode current scale. This also solves the measurement-reference problem: tube vendors match at different plate voltages, so just publishing a mA value without full test values is kind of meaningless. So our advice to “buy a medium-matched pair” is always going to be actually more reproducible, even though it sounds less rigorous.
So why no user-adjustable bias? That trades a few percent of the theoretical optimum for three real benefits: no screwdriver settings bias to 70mA because “internet says it’s better” (I do not think you fall into this category, by the way); subsequent support and repair costs; no drift-prone pot in a high-voltage bias circuit (no pot-failing-open errors, i.e., red plating) , and no service when changing tubes.
If a specific tube pair sets the bias too far outside the window I’d consider returning the tubes and getting a matched pair more in the centre of the range. Either that, or find a local tech that will do the re-bias for you.
So I think the key point here is that it is really a design decision made by Steve. For almost all matched pairs, the Power Station will work fine for many years. It’s not a high-end tube amp where every Watt and THD matters. It is a workhorse which operates in punishing environments and is made even more reliable by clever design choices such as these.