Hi,
Encoders would be much simpler.
SnapAmps can supply 12.5A cont. @ 80V you would need to determine if that is sufficient for your motors/needs. I'd recommend keeping the existing amplifiers if possible. That way you can preserve much of the wiring. Usually the Amplifiers will accept +/- 10V Command signals. You might test them by finding this input and applying a small voltage (ie 1.5V battery) to see if you can make your motors drive forward and reverse (reverse the battery).
Good luck
TK