Depending on what microsteps your xylotex is, your steps in motor tuning should be at 32000 if you have 1/8 micro on xylotex, if 1/10 you would need 40000. You should play with acceleration and speed until your motors sound good and run smooth.
The equestion(sp?) for step calculation:
200(steps per rev on motor) * 20(tpi on screw) * 8(if 1/8 microstep on drive)