| I find swapping pre tubes to have a more subtle effect on tone. Depending on the amp, some differences are easier to hear than others. For example, in my Tmax, a cheap groovetube sound more trebly, more "open" than the Ss channel but with a less pleasing sound if you use the pre and post gain to overdrive it. Put an old Mullard in there and it becomes a little darker, thicker, but with a more pleasing dirt sound. On my sm400s, the differences were less obviously. An old GE-JAN sounds a little fuller/sweeter but not by much compared to a cheapy.
Can try different stuff but if one tube has a drastic different sound than another tube, something is wrong with one of those tubes.
Assuming the same tube type of course. Some amps you can put an AT7 or AU7 in place of an AX7 to get less gain/dirt, more clean headroom. | 
