hello,
electronic circuit like :
http://reprap.org/wiki/Pololu_stepper_driver_board
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reprap.org
Please note: Pololu stepper driver boards has 0.05 ohm sense resistors instead of StepStick 0.2 ohm. A Pololu stepper driver board is a well known RepRap machine ...
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or
http://www.schmalzhaus.com/EasyDriver/
can control the steeper in a more acurate way than what you can do with an audio amplifier.
(see datasheet for more info about the way this chip control a motor).
they are inexpensive and can easily be control thanks to a line out and small electronic circuit (OP AMP)
I think you will have better result using this dedicated chip than hacking an audio amplifier.
cheers
c