You should stay with 12at7. They are designed to increase current in the amp compared to 12ax7 and 12au7 which are designed for volume amplification. Mu doesn't matter in a lot of cases when it comes to 12at7 since a lot of circuits where they are used do not amplify sound but provide more current to output tubes - thus the increase of current.
See my review here: http://tubemaze.info/on-12at7-differences-similarities/
See my review here: http://tubemaze.info/on-12at7-differences-similarities/