Hi jgd,
The tubes in your MP-1 are more than likely, the original Chinese 12AX7A's that were shipped with the unit.
My personal preference for tubes here are good quality Mullard RI's. Mullards bring out good clarity in the midrange and upper midrange frequency, and they also have a good tight low end response. Of course right now, I'm still rolling different tubes in mine to see if there is something better to my ears.
I like a lot of different styles of music, and like to play as many different ones that I can, so I need an amp that can handle that. ADA fits the bill. My favorites are Classic Rock, 70's, 80's, Jazz Rock, Fusion, Blues, Progressive Rock, Progressive Metal, Art Rock, need I go on?
You may want to look into Ruby Tubes. Many people say they are very good quality. I have those in my 3TM now, and they aren't bad at all. I'll probably roll a set of the same tubes in the MP-1 to see how well they perform there. All the preamp tubes I use are long plate tubes to keep the low end in the mix.
Harley