Jonathan,
Not sure about the EPP mode, PC was internet purchase and is a Mach3 machine, seen it run another set up similar in scale to mine.
Have checked motors, they are pretty stiff un fed but when I get volts at the motor terminals, they don't change so I would say they don't lock up. Should they?
I have 17V dc across the A - B outgoing motor feeds at driver when drivers enabled, and same at motor.
Cheers