Why did you get them from Arc, they're the most expensive place for steppers? £45 for those steppers from Arc versus about £26 on eBay (depending how many you buy).

Anyway..

10 Amps should be enough for two (must be 2 as it's a lathe?) of those motors in Bipolar parallel or series. What makes you say they're not getting enough 'juice'? Not much torque, slow, not heating up?

36 Volts is a bit low for those motors, 50-75V is much better however if your drivers can't accept that high a voltage it may not be worthwhile replacing them.