Q: What is the difference between Mesa Boogie 12AX7 and SPAX7 preamp tubes?
A: The SPAX7 is considered to be the “pick of the litter.” While an SPAX7 isn’t mechanically different from a regular Mesa 12AX7, it is substantially quieter then a normal 12AX7. Mesa Boogie tests all of their 12AX7 tubes in a test amp using extreme high-gain settings. Every now and again they come across tubes that are extraordinarily quiet, and these become the SPAX7.
The SPAX7 would be a great choice for any player using high gain for the initial/first gain stage, while the rest of the stages would be good candidates for a regular 12AX7.