I thought that people stopped using series resistors with stepper
motors years ago as the chopper drivers became cheap.  The problem with
using too high a voltage with steppers is that they will overheat when
in a locked position  for an extended period.

The modern solution is to use a pulse width modulated (chopper) driver
that supplies high voltage to the motor but drops this voltage by
reducing the on time portion of a square wave fed to the motor when the
motor is at rest.  You can buy ready made controllers that do this or
you can make your own with a combination of the good old reliable L297
driver and L298 H bridge made by SGS Thompson.  I am not familiar with
the design of the MIDster so I do not know if it uses step or phase
signals, but it should be easy to modify the software to provide the
step and direction signals for the L297 driver.

You will be amazed at the work a small stepper can do with one of these
drivers or even better with a more modern commercial ready made unit
that shouldn't cost more than about $125.

Spencer Chase
http://nt.mcn.org/spencer/