Stepper Motor current rating and microstepping

A properly designed chopper PWM current regulator will let you run the motors much faster while maintaining torque than other other solution.

As the rotation speed and thus commutation frequency of a stepper motor increases, the winding inductance begins to present a larger and larger reactance, and limit the current which will flow at rated voltage. The solution is to use a power supply much above the rated voltage, but use a chopper current regulator to prevent overheating the motor or damaging its permanent magnets from excessive field strength.

The power resistor and higher voltage supply method has enough advantage over "nothing" as to have been used in some large stepper motor systems before choppers became affordable, but is a far inferior solution to active current regulation.

In terms of available pins, remember that once you start putting any ICs on a board, putting an additional ATMEGA xx8 on a board becomes fairly economical, too - you don't need a whole Arduino, and the prices for what you do need range from about $2-$4. It's not uncommon if someone is going to the trouble of marketing a stepper power amplifier module for hobby/robots use to put a microcontroller on it and give it a serial command interface instead of raw step/direction per motor.

I actually figured it out. The stepper motor itself cannot regulate current and so has no way of stepping down to a safer level. It is the job of the motor controller to provide safe current level to the motor. Since I was powering the motor externally, the motor controller may not have been providing proper current regulation.

And since the coil resistance is so low for these motors, anything more than ~1.5A will start to cause problems.