Increasing the current can damage the motors, so don't increase the resistor value. The G540 has a current reduction when not moving. If you move the motors, does it then have more torque after it stops?
Something is wrong with your setup. The Xylotex has a max setting of 1/8 microstep mode. The G540 is fixed at 1/10. So if your Xylotex was set to 1/8, with the G540 you should have 10,000 steps per unit. There's no way the steps per unit can be less than the Xylotex with the G540. You might want to call Gecko with your concerns. They should be able to get you set up pretty quickly.