It depends on how old. After the death of most major tube manufacturors in the late 70's and 80's the tubes being produced from then on are significantly lower in musical, build, and consistency of production quality as well as much less longevity. Amps built after the golden years of tube manufacturing are definitely hampered by lesser quality tubes they are forced to use. Right now the tube market is improving with new production tubes starting to get better in quality, but frankly, there still is a long way to go. And some of those nicer newer tubes are not among those sourced for amp production by Mesa. That is why some members of the forum are so heavily into obtaining reasonably priced, used vintage old stock or unused, rare and expensive new old stock tubes or getting their new tubes from dealers outside mesa from sources such as Eurotubes and Doug's tubes, to name a couple of ones often mentioned here.
As far as your amp sounding different with the new tubes, yes, it will, and possibly significantly different if those old tubes (particularly the power tubes) have had more than a couple thousand hours on them.
I'm guessing that getting the info straight from the horse's mouth would be the best way to answer your question regarding whether the new tubes available today will be the same manufactoror(s) as the originals in your amp. Mesa answers phone calls. Peace.
