I found this chart pretty informative if you're looking for headroom. The 12AU7 makes a huge difference in volume.
Gain Factor
12AX7=100
5751=70
12AT7=60
12AY7=45
12AV7=41
12AU7=19
Substitution
12AX7 - ECC83, 7025, ECC803, E83CC, 6681
12AT7 - ECC81, 6201, 6679
12AY7 - 6072
12AV7 - 5965
12AU7...