Forgot all about reporting on this. Sorry about that. Good catch, Berndt.
Actually there's not much difference. Doc Peppard explained that since the CF circuit uses 100% negative feedback, there would be little difference between brands. In the Magic 5B the 6R7's are the output tubes in the circuit, the input is split between each side of the center 12SN7 triode.
Overall, the metal tubes (RCA, Visseuax and Neotrons) seem to be a bit brighter. The Nationals are quite smooth but a bit rolled off on top. For me, my favorites seem to be the Sylvanias and Philcos. They have a nice combination of clarity, detail, and bass without being too bright. This is all specific to my system of course (VTL's/Magnepans).
There seems to be much more sonic difference between 12SN7's in the SRPP circuit than with the 6R7's in the CF circuit. I tend to use the 6R7 circuit most of the time--it is more fuller bodied with better bass than the SRPP circuit in my system.