Both tubes are electrically identical. The "A" is a slightly more rugged construction which results in lower output microphonics. The "A" designation is more for application purposes, designed to be used in a series heater circuit of tv's, radios and other older electronics. But for modern audio, the 12V circuits are not in series and therefore should make no difference which designation you use. As for sound, tubes are different and I cannot really say if the "A" is better/worse for a particular application for a given tube.
Difference between 12AU7 and 12AU7A
Anyone out there know what the difference between these two tubes is? All 9 pin mini dual triodes suppossedly can be rolled into the voltage amp stage of my Llano Trinity 300.