The tonal differences in 12AX7s are pretty subtle unless you are overdriving the input with some sort of pedal. A Sorado doesn't have a Master Volume or the extra gain stages to create distortion so the 12AX7 is just used as a clean voltage amplifier up until the phase inverter is overdriven. I got a "sampler" of 5 different 12AX7s from a guy on ebay. There wasn't any marked difference between any of them. Slight gain difference of probably less than 1dB and a little bit of distortion but nothing to make me not want to buy the one with the best price. As far a microphonics they all vary. I've gone through 20 of the same type and picked 2 or 3 that were low enough for a Rivera guitar amp. They weren't really low, the ringing was just too high frequency to hear.
