Which SRM-1 is the best?The C series is by far the best. If you get a chance to audition an A series side by side with a C series, the differences will be very easy to hear. Unfortunately, it really is that big. For someone looking for a used SRM-1, I would never recommend anything older than a C series. Unless someone is willing to personally modify the device and painstakingly upgrade the 30 or so resistors inside. (Lots of time and money).
Speaking of C series, I later found out that the highest serial numbers made by STAX before the SRM-1 was discontinued are the ones ending in the 6000 range (i.e. C6xxx). The 6000 range featured a further upgrade to the resistors which now go beyond the level of Roderstein. Unfortunately, I don't know for sure which new brand of resistor was used.
However, I clearly remember 10 years ago, when I was deeply involved in high end audio, I auditioned side by side a C5xxx with a C6xxx and I could hear a significant difference even between the two different C series M1's. The newer model had much less of that electronic haze/electronic texture that many people have reported when listening to the M1 driver. The differences between an A and C series is far bigger.
When buying an SRM-1 used, do yourself a favor and look for the C series serial number.