That's not necessarily true. Using a tube with lower gain is akin to using a flea-powered SET amp instead of high-powered Solid State amp. 12AU7 and 12AT7 are different tube families with a different sonic character. Adding something to the signal path with the intention of limiting the voltage (and probably the dynamics) - that's not what I tried to achieve. I just thought less gain might mean more music. The 12AU7's sound is so relaxed, soft and free of any edginess that it's hard to say whether the amps dynamic impact has been compromised or whether the sonic signature is just less busy, whether there is simply less artificial high-lighting of transients. At this point, I'm leaning toward the latter explanation, but my impressions aren't definitive.
And there might be another tube to try: the E80CC which looks like the 12AT7 on steroids, it's a lot taller. Pin-out and heater voltage are identical to the 12AU7/12AT7/12AX7 group, but it has twice the heater current. As far as I know, anyone at Audio Asylum who replaced his preamp's 12AU7 with the E80CC has reported improvements. With its gain of 27, the E80CC might have a slightly more dynamic sound than the soft-spoken 12AU7. I don't know.