Tubes in the preamp's power supply -how important?

Yes, agree with the above posters. They make a big difference. Try the NOS varieties and go fro there. All sound different and one will suite your system best.

Huge indeed. My experience reflects the same with Aesthetix and even more so with the Counterpoint and Aria products. I recently replaced a Sovtek EL84 in a rebuilt Counterpoint SA-2 with a Valvo EL84 and was mightily impressed with the added smoothness in the mids and greater tonal coherency on the top. Surprisingly, the Telefunken EL84 was rather dull here. It pays to try a few different brands to determine which one "locks in" for you.

Depends on the design of the unit. In Audio Research, Aesthetix and Allnic, three that I have experimented with, the effect is huge.