Hi Jim, I am well aware of Chris Johnson and TPC as I have owned SFI gear for 6+ years. I will Email him about the actual job of each of the various pairs. But if you are right and the rear slots are the most important then you are saying the Buffer stage is more important then the gain stage. I hear differences in the slot movements but im trying to maximize the importance of a single pair of tubes in each piece as I don't wish to spend that kind of money on NOS. I find the sound right now using the gain slots for my better tubes to be much more dimensional sounding. I am using EH 6922's in the 2 pairs and in the gain slots a pair of Siemens 7308/E188cc's gold pins in the other slots. I have a host of other tubes that ill be playing with in due time but right now I am enjoying this setup.
Greg