12AX7 or 12AT7

Check out this URL:
http://thetubestore.com/gainfactor.html

The U has less gain and lower plate impedance than an X or T, about 1/3 the gain of the T and 2/3 of the plate impedance, about 1/5 the gain of the X and about 1/10 the plate impedance. It has about the same current capabilities of the T.

Unless you really understand the characteristics of these tubes and the topology of your circuit you are generally better off sticking with the original tube type.

I accidenty had them switched on my Jolida 302b. It worked, but after I corrected them it was like a new amp.