My favourite affordable (relative term I know) 12AX7s are; Amperex, Telefunken and Mullard. Sure they cost more than a Sovtek, but in the context of the price of an amp the money is very well spent. In my system I find the Amperex is best, having warmth and the best dynamics. The Mullards are a little relaxed. The Telefunkens are often considered to be the best but the better dynamics of the Amperex wins out in my view. You do not need to worry about swapping one 12AX7 for another - issues of biasing only apply to power tubes.
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