Hi John,
So far the Vipers have been behaving well. It is a little tricky to get everything hooked up properly, like the phase of the encoder and motor polarity, etc, but once it is all figured out they seem to work very well. The motors are Gettys Fanuc, and have 2000 line differential encoders.
If your drives, or "amplifiers" are working well, you may consider some sort of out-of-Mach motion control board. I've heard there are some that just get commands from Mach and close the encoder loop etc all in hardware. Not an expert here, and I knew Mach works pretty well at motion control so I went the digital drive route.
Dustin