Best tube for Cal Sigma 2

Need to replace my Sovtek for something a bit better, any suggestions would be greatly appreciated. Thanks
yugo ei12ax7 - very neutral and transparent - i use it in both the sigma and alpha.
I've remember reading that alot of people use 5751 tubes instead of the 12ax7 tube. One gentleman suggested using a Raytheon 5751 tube instead of a 12ax7 tube. The Raytheon apparently is very smooth yet extremely detailed.