If you have not already purchased the amp I recommend
(1) finding one that has already had the tri-tube mod, or
(2) negotiating the price downward to reflect your cost and down-time getting the mod done.
With the tri-tube mod you can run 5881/6L6's (but only 6L6GC's as there is too much voltage for GB's); EL34's, KT90's; E34L's; and KT99's. It is my understanding that there were *some* Barons built to take 6550's but I believe they only went into one pair of sockets. This is not a straight drop-in, adjust the bias proposition like the other tubes I mentioned. Big time arcing would occur if voltage isn't reduced for the 6550's. Check with Mesa to confirm.
The biggest and least expensive imrpovement you can make is to replace the Chinese 12AX7's with good NOS tubes. However, even this will not entirely compensate for what I find to be the edginess of the Sovtek 5881's. Not a bad tube, just rough around the edges. If you have money to burn, NOS RCA 6L6GC's are great but these will run upwards of $80 per tube for a matched set.
I have gone with triple point matched trios of the Svetlana EL34's and like the effect very much. In two-thirds pentode, they are much smoother than the 5881's were in two-thirds triode, with the increased detail and more open sound that more pentode produces in my system. The 5881's seem to have more gusto and more bottom end, but that isn't much of an issue to me since my main speakers are crossed over at 100htz with a fairly steep slope. Cost was real world at $20 per tube. You can get less closely matched pairs for the same cost most everywhere.
This is a powerful and incredibly flexible amp in stock form, but a little rough in my opinion. Until I went with the EL34's I was able to effectively compensate with triode/pentode and negative feedback adjustments for different quality source material. This was OK, but the EL34's bring this amp up to a significantly better level of performance. Good luck.
