Are you sure you can mix anything in there? I went and had a look at the specs and it says they're convertible from 6L6 to EL34. Now i know the tubes you're talking about are in the same respective families, but is there a section in you manual that says "anything from either"?
Tubes aren't just tubes, especially power amp tubes, a lot of them have different electrical specs (which fair enough, I'm not up on). Just advising a bit of checking first?