Phonoland

The following tubes are in the MRA3-L6 amp of my M100C. Can a substitution be made for any of these tubes that would perform the same or better?
6L6-G, 5U4-G, 6SL7-GT, 6SK7-GT, 6SN7-GT, 6J7.

Also, what is the significance of the suffix letter (B) after a tube's main identification? For example, 6SN7-GT vs. 6SN7-GT-B.

Thanks,
Steve

Steve,
IMHO, Nope--
"B" or "C" indicates a later (improved ?) version of the tube--Generally speaking, the later version can be sub'ed for the earlier version, without problems. I try to stay away from using an early version where it calls for the later one-- Ron Rich