Tube rolling with my Mark IIc+....Suggestions?

I'n thinking about auditioning some different tubes for my Mark IIc+ DRG. It currently has a matched quad of JJ 6L6's in the power amp and Mullard 12ax7's in the preamp. The amp sounds FANTASTIC as it is but I'm just curious. I'm currently trying to track down some STR-415 6L6's for the power amp. Not sure which direction I'd take with the preamp since it already has some killer tubes in it. What have you folks tried with your Mark series amps?

My favorite tube combination is probably NOS Chinese Military Beijing 12ax7's and Sovtek 5881's. The Chinese 12ax7's have a sweet, dynamic overdrive and the Sovtek 5881's sound thick. The 5881's are also very durable and can handle the voltage when used in a c+\III+ with the 105 transformer.

You've got a good thing going with the Mullards. My favorite combo has been Mullards with Philips 7581a power tubes. More specifically v1-F91, v2-161, v3-I63, v4-I63, v5-F91. Tungsram ecc83 also works really well in the amp and for a time were the stock tube for the C+. Amperex are similar to the Mullards, but I'd say a little less middy and a little more grindy. RFT is also nice in the gain positions. =C= is a good option for the big bottles and if you can find them Sylvania 6ca7 is a nice swap to change the pace a bit. But a good set of Sylvania 6L6gc with your Mullards are gonna be hard to beat. Biggest thing to know going in is that no matter the tubes, a C+ will always sound like a C+, but in a critical listening senario (in the bedroom) you will hear the differences the tubes impart on the amp