Today I received my third pair of "Sovtek" 6C45Pi-E. It`s a new production tubes, former pairs were NOS from 70`s/80`s. This pair is well matched and closer to the specs, Ua is about 160V, and µ=43/45. I tried changing Ia through the tube(s) in 1mA steps. IME - lower current "sounds" darker and less sharp/rough. About 9-9,5 mA sounds best for my ears, although I can "swear" that I heard a little sound differences between the 6 tubes I have, too.
We can (safety reasons) "drop" the B+ a bit, on about 400V. I didn`t hear the difference.