I just thought I would share my experience. I recently decided to try a JJ 12ax7 in my QuatraValve and thought "hmm, why not try it in my RAH350SMX head as well?" I have read so many places that a preamp tube swap on a hybrid head really doesn't make much of a difference. Well, I disagree when it comes to TE heads! WOW, what a difference one little tube can make! My tone became much thicker and more round. It also fattened up the bottom end alot. One other little side note, my already excellent sounding harmonics somehow sound even richer. Wow, what else can I say? Any other Trace Elliot or other hybrid owners done a tube swap with noticeable improvements?