The 12AU7 clipper circuit takes the place of the modulation limiter in this radio.
Oddly enough, the lower the gain of the tube in this circuit, the higher the audio level you'll get from it. The 12AX7 used in the mike-preamp stage has an amplification factor (mu) of about 100. The mu for a 12AU7 is around 20.
Don't know of a compatible tube with a lower mu.
If you put the higher-gain 12AX7 in that socket you get less audio, not more. I know, sounds backwards. You can try making the resistor between cathode and ground a higher value. Should get you a little more output from the 12AU7.
Are you trying to drive the 6BQ5 modulator harder? The stock circuit will hit it pretty hard with the mike gain turned up.
Or are you trying to modify the tone balance? A clipper circuit requires filtering out bass tones. If you're looking for more bass, bypassing the clipper will be necessary.
73