ok about balanced and not balanced issue is only a taste matter, for example regarding power tubes I ever recomend well balanced tubes better than a high tech bias adjustment.
balanced triodes are for attitudes, a non balanced triodes gets a phase of the waveform a bit stronger than the other phase, that little difference is amplified by the tubes but not too much the necesarly to just ear a curious distortion, that distortion is totally allowed on guitar amps due the distortion increments the first odd harmonics getting your amp a bit better for soloing, thing that for example instruments like clarinet has the odd harmonics stronger than even and clarinets are using as solo instruments on the orchestrals.
Well now regarding about substitute 12at7 for 12ax7 tubes I´m a real total hater of that kind of substitutions. I consider the 12at7 out of the family of the 12a*7 due all the tubes from that family could dissipate 1 watt and the 12at7 2 watts, ok what does it means, well it means that 12at7 is able to flow the double of current of the all other tubes, for that reason when a designer draw a circuit he is thinking about the tube properties he goes to the datasheet and fixed all the parameters, if a circuit is designed to flow 10 mA with a 12at7 and you replace it with a 12ax7 it will be totally overloaded, for that reason I ever recomend ever to replace tubes with the same kind of tube, I know that there are people that try it and the tube works, ok my car tires are 205 r15, I could replace it with all the r15 wheels, so if the car designer designs the stability of the car he was thinking about a 205 r15 if I replace it with a 175 r15 did you think that the car will be working as the designer thought???? even both tires are r15 and they fit well on my car???