Lance,
I know in past preamps I have had very good results with the Tung sol black glass 6sn7,Slyvania chrome domes and 3 hole bad boys,treasure cv-181-z and the new production Sophia electric 6sn7's but had terrible results with the new production Tung sol 6sn7's.
The MZ2 is probably a tube rollers delight,sounds like it responds very well to diff. tube combos.
I still have my Valvet soul shine that was a Little better than a modwright Ls-100 that I sold,both very respectful pre's though.
I see some guys are having very good luck finding and using the 12sn7's It's a nice feature that the MZ2 can use both.
Kenny.