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?