Tom - so far, so good. I'm currently listening with all of the stock tubes to get a feel for things, but have the NOS RCA 5751's & Sylvania 12AT7's ready to roll. Surprisingly, the amp sounds really, really good with just the stock tubes... a very pleasant surprise ;o).
The MC275 with the stock tubes is lacking the detail & refinement provided by my previous amp (Plinius SA-102), but it's noticeably more musical, natural and robust sounding without being in your face... I'm really loving it! Will be interesting to see if the NOS signal tubes take the sound to a new level.