Seeking tube 12AX7 recommendation for a Conrad-Johnson Premier 3

Mullards are pricy. The longplate square getter is the best I've heard, but it's about $400 - 500 per pair. Check out all the versions. The CV4004 is a magical sounding tube.
Also, look at Brimar. Made in UK as well, more extended and dynamic than a Mullard. 

Mullards give you a warm timbre, rich smooth top-end, but very little transparency. Mullards work great when combined with a different tube brand in the next stage.
Check to see what the 5751’s are.

I highly recommend RCA 12ax7 1950s blackplates for warm open sonics with liquid mids and smooth highs.
But it depends what type of sound you seek. For transparent, out of this world 3D imaging...Telefunken. For open, realistic, and excellent  dynamics...Amperex.

This trusted dealer explains the different types of 12AX7s very well.

http://www.audiotubes.com/12ax7.htm

@arturos73 ,
Tubes branded as Hammond are good quality since they need to perform well in their organs.

According to the manual, the 5751s are used in both the line stage and phonostage as cathode followers and anode followers. They will have less influence on the sonics than the 12AX7 gain stage. But even cathode followers are worth rolling.

Tube complement:
V1,V2: 12AX7
V3,V4,V6,V7,V8: 5751
V5,V9: 5965

I like to pair a neutral tube with Mullards. RCA or GE blackplates are excellent tubes; open, smooth highs, but will add some warmth to the mids. IMO, RCA will add too much warmth.

My choice would be a Sylvania 5751 Triple Mica blackplate Premium. It’s a neutral and dynamic tube with airy highs. 
I see that Brent Jessee carries 1950s Tung Sol 5751s. Not as neutral as Sylvania, but open and dynamic.

Unfortunately Amperex does not offer a 5751, only a 12ax7 which CJ does not recommend as a substitute.