Upscale's suggestion is the right one. I had two matched pairs of Mazda Chrome Plates and matched sextets of Svetlana 6L6GC's in my Baron before I traded it in.
At the time I traded it in, it was sounding better than it ever had in the past and I owned it for about a year and a half. During that time I'd tried all sorts of power tubes (Mullards, Teslas, Svetlana EL's) but at the end of the day, the Svet 6L6GC's sounded best. Had the best bottom end, and slightly rolled off top, which is what I like (no glare). I'd also tried too many combinations of 12AX7's than you have time to read about (Tungs, Tele's, Mullards, etc. etc.) None of the previous combinations were as quiet and transparent in the Baron than the Mazda's.
IMO if you've got a Mesa Baron, you must get Mazda Chrome Plates for the pre driver stages. You will not recognize your amp's sound characteristics after this serious upgrade. You'll think someone switched amplifiers on you AND you'll be glad they did !!
Change the power tubes all you want for subtle changes with varying lesser degrees of improvement. Change all four of those 12AX7's and get ready for a much more dynamic and involving experience with your Baron.
