How important is the pre-amp?

I disagree that the preamp’s sonic performance is always tightly correlated with the price.

The issue with transformers is loading and inter-winding capacitance. All transformers have inter-winding capacitance. If the transformer is not loaded correctly (usually too lightly) the capacitance may play a role causing the frequency response to be less than flat. This coloration can be easily heard.

Transformers are called that because they transform impedance. It goes both ways; you're not insulated on the input from the output. So if the output load is reduced, so is the impedance on the input side.

What happens when you have a transformer with multiple taps used as a volume control? Since you are working with a variable turns ratio, to keep the transformer linear you need to have the correct load at both the input and output. You can't, as a designer, expect to always see the same source impedance and you certainly don't see the same load impedance since all power amps are different in that regard. On top of that you have the moving target of variable turns ratio.

So you'll need a lot of switch contacts, not just the ones to change taps but another set with resistor to correct for the changing load impedance as the turns ratio is varied.

This has to be done right, else the unit can introduce coloration.