Surfmuz, You wrote, "…12AU7, 12AT7 and 12AX7 are pretty much the same tube with a little different gain settings." As mulveling already intimated, this statement could not be more wrong. The only thing these three tube types have in common is that all 3 can be run off either a 6.3V or a 12.6V filament supply, depending upon how the socket is wired, and all the other pin connections (to grids, plates, and cathodes) are cross-compatible. But that does NOT mean you can freely switch among them. Any listening impression you get or got by substituting one for another type is as likely due to harming the performance of the substituted tube by subjecting it to too much or too little current, or too much or too little plate voltage, or usually both, as it is to any inherent sonic differences between the types. As a general bit of advice, I would suggest that if you don’t understand tube electronics, you ought not to replace one tube type with another different tube type.
As to the 12AU7, I dislike the sonics of this tube when it is used for gain. If it is used as a cathode follower or as a phase splitter (in an amplifier), that’s OK, because those two functions do not add gain to the circuit. 12AU7 can be replaced by 6CG7, 6FQ7, 12FQ7, or even 6SN7, which requires a change of tube socket, but only if you know what you are doing and can re-wire the existing 9-pin socket (or replace it with an octal socket for a 6SN7) to suit any of those tube types. All of those tube types are very similar electrically, if not identical, to the 12AU7. But don’t just plug them in; you need to re-wire the socket.
Mijo, ARC products are notorious for driving the tubes "hard", which causes them to wear out faster than typical. So you are well advised to do what you do.
