Does anyone know someone who can modify Mesa Baron

Talk with Mike at Mesa. The most important upgrade is to replace the 10,00uf 16V brown Sprague electrolytics on the audio board. Highly prone to leakage.
Had my amp set up for EL34s many years ago, much smoother than the 5881. Also replaced the .1uf caps on the outputs to Hovland Musicaps and will soon go to Mundorf silver and oil caps. The original Mesa tubes tended to be a little bright in combination with the orangedrops.

Magnumip205, I also prefer EL34's in the Baron. My original 16V caps did leak, and Mesa replaced them. Would you consider upgrading the replacements Mesa used important? They seem better than the originals, but Mesa would never use an expensive "audiophile" cap. re: the eight .1 uf caps, did you upgrade those from the original Sprague 715's, or had you tried the Wima upgrade Mesa offered?

I have had one since 1998. Mine was modified by Mesa to be a tri tube version with WIMA caps. The speakers are Soundlab Dynastat. Tubes are JJ E34L all matched to 1ma, drivers and splitters are Mazda silver plate or (sometimes) Mullard m8137.

This combo is outstanding. I have had musicians and singers that have appeared at the Kennedy Center and the Metropolitan and 9 of 10 said it sounded as if there were at a live performance.

Since you are in California, Mesa might be the best idea...they are expensive however. The reason I had mine done by them was when I bought it from a California dealer they had already made the arrangements for the work ( otherwise I would have done it). I have been restoring vintage gear and nome new products for over 30 years (and have done work for Joe at Tempo Electric), and intend to rework the amp very soon.

What I will do to it includes, hexfred diodes for the 10 cent 1N4007 line up, Muse electrolytic bypassed with small oil caps and some ATOM caps (if I can't fit film caps in), dump the WIMA and Sprague 715 (which are NOT audio caps)and replace with Obbligato or Ampohm, and resistors in the signal path get replaced with bulk metal type. The output tubes a RCA 12 matched Blackplate 6L6GC.

I could modify yours, but as others have said here, you need to listen to yours first...it may just be what and how your want your amp to sound. Then try a Mullard 12ax7 or a Raytheon/RCA blackplate 12ax7 (very similar to a Mazda in sound) and see what that does. It is very important that your outputs be matched to within/no more than 2ma total spread per set of six.

Farmersre. Truly sorry for your loss. Astrosytomas are mean SOB's! I will give you my 2 cents. Just listen to it for a while in triode mode. Don't just rush right out and have someone mod it. Everybody has their own idea of what it should sound like. Your ears, your audio system is truly unique to you. What sounds good in one person's system might not sound good in yours. I repair ARC in warranty, and sometimes update as well. I'm very careful about mods. After listening to your amp for a while with different material try and determine what you like and what you don't like about the amp. If they're sonic issues it may not be the amp. I also believe calling Mesa is a great idea. Talk to them about it. After all, they designed and voiced it.

Best Amp I ever had! Leave it stock/Tube roll if you must! I have a Tigris and love it as well!
Cheers!