I'm in the process of retubing my 5150 starting with the pre amp, and I'm debating playing with the power amp as well. I have Winged C 6L6 in there right now, but I have a few Sovtek 6L6 WXT that came out of it when I got it. I only have 3 because one broke and that's why they were replaced, so I can't just put the Sovteks back in to see how they sound. However, I'm thinking of being creative with it and pulling the outside two =C= and replacing them with the Sovteks. I don't think that would cause any damage since it's a push/pull system, but I thought I'd ask anyway.