12AX7 tubes for Counterpoint NP400

I have a Counterpoint NP400 hybrid amp (200x2 into 8 ohms), which was recently given an "ultimate" ugrade by the amp's designer, Michael Elliot. I am kind of new to tubes, but am hoping to play around with some different tubes. Any suggestions for good (great) tubes to try out. Are there any concerns with various tubes I need to be aware of? Can I just drop them in, or do I need to rebias the amp? Any advice would be greatly appreciated. Thanks