First of all, forget potentiometers. You would need f**cking great big wirewound ones to handle the power.

Also for 18v motors I would use an 18v supply. PWM controllers give you full voltage, but vary the on/off ratio. This means you get full torque, but the power is in bursts, which regulates the speed. If you are using 24v then you could burn out the motors.

I look forward to the time you show your balls to the forum !

Cheers,

Rob