The European versions of the 12AX7(ECC803/ECC803S/E83CC) usually had the gold-plated pins, thicker or triple mica, matched triodes and were better built for longer life/lower noise(the "S" indicated "select" and they were especially quiet and balanced). You would hear an improvement with these tubes, but- if you enjoy the added warmth that the Mullards provide, you may miss that with some of them. The 10M Mullard is built to the same standards(and even more rugged), will provide a platter-full of improvements, and still possess the sound you're enjoying now(well- maybe a bit less microphonic).
Mullard 12AX7 10M gold pins vs. steel pins
I'm considering trying some different NOS tubes for my Aesthetix Calypso (currently using Mullard CV 4004's with good results). Does anyone know if there really is an advantage to the sound quality with the gold pins vs. the less expensive steel pins?