Please go on an auction site and look at the prices of 12SN7s as compared to 6SN7s. Sylvania, and I agree they are some of the better tubes of this type, made both 6SN7s and 12SN7s, as did RCA, Tung Sol and others.
A 6SN7 chrome dome Sylvania is the same construction as a 12SN7 chrome dome Sylvania, just the filaments are different. To look at them they are identical. However the 12SN7s are available at pennies on the dollar as compared to the 6SN7s. Same with Tung Sol 6SN7s and 12SN7s. Same construction, with the exception of the filaments, yet look identical. For the Tung Sol 6SN7 you'll pay $50.00 per tube, and for the 12SN7 maybe $5 to $10 dollars per tube.
If one is intending to shell out a couple of hundred dollars for a set of four 6SN7s, I am suggesting one consider changing the 9 volt filament tranny to a 15 volt tranny ($30.00?) and R21 from its 6 volt value of 470 ohms to either 910 or 1K instead. This will set the filaments at 12+ volts and then you can use 12SN7s. I actually went with 100 ohms on R20 (normally 120R) and 1K on R21.
As an example I recently picked up a quad of brand new in the box, never used, RCA coke bottle
12SN7s for $25.00. It would have been at least four times that for a set of 6SN7 coke bottle RCAs.
Something else to consider: the voltage drop from 15 to 12 volts is a lot less than the drop going from 9 to 6, so it ends up being more efficient with less waste heat.
I can detect no difference in sound between a same type 6SN7 and its corresponding 12SN7. If anything the 12SN7s sound better as you can find them new and unused, unlike most good vintage 6SN7s.
Just my two cents. Not intended to offend anyone in anyway, but offered out of my experience as an alternative to consider.