Hi Don:
This post is a few months old, but I just read your questions. I have an older mill, with probably the same drives and the very same Electrocraft servo's. The driver/Amps were made by WestAmp, now out of business since I believe 1992. This type of control is Analog +/- 10v with encoder feedback. In the day, these were very good drives, and would hold position well.
The big problem is getting the drives repaired when they malfunction. Even though it might be a single resistor, easily replaced, finding exactly what is wrong can be very difficult without the proper electronic equipment. I have been able to find a few used drives on Ebay, but that is hit or miss.
The mill discussed now uses a dspmc controller by Vital Systems, and it runs the older drives perfectly. I did have to change the encoders to the AMT differential to work with the controller, but is runs better now than the original controller.
Since doing the retro-fit to the older controller, I have since learned of a newer type of Servo motor/drive that is far better than anything I have yet seen, and it uses step/dir for an absolutely solid, precise control.
If I did another machine, I would definitely use the Clearpath servo's by Teknic. There are Youtube videos showing these motor/servo's with Mach3.
John