Any 12Axx tube is made as a 12AX7, then it is tested and rated as to how much gain it creates. A 12AX7 gives 100%, the xxAU7 is 60% and so forth. the manufacture of the tubes in question my have a differant arangement inside the tube but the ratings are the important part, THEN the innards.

I would try the GTubes smooth plate or the 'M' series, but I would keep the numbers the same unless it is to the europian version (12AX7/ ECC92).

A great tube will make a HUGE differance in the mic performance.