Why do you just assume that changing tubes will ameliorate your problem? For example, first of all, what evidence suggests that the problem lies in your phono stage? Bass boominess could be the result of a mismatch between cartridge compliance and tonearm effective mass, if the combo of those two results in a resonant frequency that lies too close to the audio spectrum (commonly taken to be 20Hz to 20kHz). Or the problem could certainly be in your speakers.
If this is an MM phono stage, then it is likely that the 12AX7 provides all or nearly all the gain in the system, whereas the 12AT7 may either be a cathode follower, which adds no gain, or a second gain stage with lesser mu. If you change the 12AX7, the phono will likely sound "different" if not necessarily better.
If this is an MM phono stage, then it is likely that the 12AX7 provides all or nearly all the gain in the system, whereas the 12AT7 may either be a cathode follower, which adds no gain, or a second gain stage with lesser mu. If you change the 12AX7, the phono will likely sound "different" if not necessarily better.