My favourite combination in the MP3 (2 12au7, 2 6sn7s, and a 12at7 not in audio circuit)is the 1940/50s RCA 12au7 Blackgate combined with the Sylvania 6sn7GTA (not sure GTB would make any difference). I tried the RCA Cleartop and Siemens 5814 which seem some what similar to me, compared to the Blackgate 12au7. I use the pre with the M60 amps and I think the 12au7 lends a touch of warmth which is a nice touch with the neutrality and transparency of the amps and my Merlins. The RCA 5814 seems very similar to the Blackgates. My combination definitely performs very well for my music and ears, but I don't doubt there may be even better combinations out there, but I'll stick with this for a while. P.S. I use RCA combined with Sylvania GTBs in the amp - 2 RCA in front 2 Sylv. in rear slots; I prefered this to all Sylvania. This being said, the Atma-spheres sounded great with any of this combinations, it is just a bit of spicing to a basic recipe.