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
I have 12ax7 in my phono stage.The sovtek Lps is a quiet tube but a little bright.I had them but bought Telefunkens which are even quieter and sound MUCH better.I bought 2 from Kevin Deal @ upscale audio.$35 each.No you only bias ouput tubes 6550,EL-34 etc.
contact Christian at Electron tube valve.He will be very helpfull.www.electrontube.com
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.
The Sovtek 12AX7 is an okay sounding tube at best. If you are looking for good advice and very nice pricing, try Triode Electronics in Chicago. (773) 871-7459. Their hours are quirky, so keep trying. I have been buying vintage tubes from them for years.
I don't know about the counterpoint but I use Special quality Mullard M8137 box anode valves (Tubes)in my Art Audio Tempo Mono blocks,Art Audio VP1 Pre Amp and Sound Audio (from Hong Kong) phono stage with great success. The M8137 is a SQ version of the Mullard ECC83 (12ax7) and was produced by Mullard to the highest military spec although it was available of the industrial market. I have a Mullard Tester and the M8137s have perfectly matched halfs remarkable!! I also have various 12ax7 Telefunkens and to my ears,and importantly in my set up, the M8137s do sound "better" I would also recommend the sister Mullard M8136 which is the Box Anode High Spec ECC 82's (12au7).Look out for the Brimar "Trustworthy" range that is often overlooked again Industrial / Military spec and very fine performers. Hope this helps.
The comments posted here already state what I would have said. The Telefunken, Mullard and Amperex tubes are all wonderful choices. However, one option, often overlooked is the black plate RCA 5751. This tube was designed for the US military (like the JAN Amperex) and as described by Redkiwi, is more dynamic. The RCA 5751 is extremely low noise, very clean and clear like the Amperex, but with greater weight, more like a Telefunken. The good news is that all of these are better than the Sovtec, and you can swap between these premium choices to arrive at the sound that best suits your preferences. The other pieces in your system will greatly influence your decision, so consider trying several, to find what is the ideal match. Personally, I keep all of these in quantity in my tube supply bin. What may be first choice in one piece of equipment, will not have the same magic in another. Consider also, the fact that many of these tubes are becoming harder and harder to find, so if you gamble and buy several of these recommended here, you certainly will never regret it! Imagine having to use "second choice" among these tubes, and then imagine instead that you have only the choice of buying what is being manufactured today. Best wishes, tell Mike Elliott I said hello.