Hi, I do sell vintage tubes and have sold tubes to several MPA Gold owners. There are several excellent choises for this unit but it all comes down to preference. Some like the detailed and airy Telefunkens. Others prefer the smooth, dark mullards. RCAs impart a nice "tubey" vintage sound. And, there are many others.
Prices have a wide range; between $30 to 100 per tube.
All the tubes I sell will offer more detaiil, more pleasing harmonic overtones, longer live and superior overall sound quality to stock tubes.
If there is a certain tone you're after, or if there is something specific that you don't like about the way your MPA Gold currently sounds, let me know and I can make some specific recommendations based on that.
Regards,
-Bowie