OK Dan. Since no one who really knows the theory has stepped up, let me give you, from my layman's perspective, the rationale here.
The two tubes to the left of the amp, looking from the front, are the crucial ones for sound, as they are the ones which actually amplify. Top is left, bottom is right. The two to their right are not as critical, so any good pair of fuctioning 6SN7s will suffice if they are reasonably well-matched.
The tube to the right side of the chassis is the rectifier, once again not a critical part of the signal path.
So the idea is to use your best matched pair in the first two holes on the left. Jim McShane supplied me with excellent new TungSols in all four positions. I decided to try some NOS and used US-made tubes to reserve the new ones, and read somewhere about prioritizing in the manner described. The new TungSols are fine, bright and solid choices. The old RCAs are a bit on the dark side. The Sylvanias are right in the middle, as are some NOS TungSols I lucked out on from EwBay.
What will satisfy you is going to depend on the rest of your equipment and your taste.
Someone who really knows what they are talking about can take it from here. BTW I got 8 RCA 6SN7s from the aforementioned auction site, and they were dirt-cheap - about $4 apiece, and tho I don't care for them in the gain position, I will never run out. Can't remember what the other old US tubes ran, but it was no more than $25 each. So you won't need a second mortgage on your home to start the tube rolling. It's addictive. And it forces you to really learn to listen for subtleties in sound which you may have taken for granted.