Yes, that could well be an issue. I advise against running either board at below its minimum rated voltage, because the chips in those boards need that minimum voltage to guarantee correct operatio
Bear in mind that both the boards you refer to are constant-current drivers, so you can turn the voltage up to 8.5V or more without damaging your steppers
provided that you set the driver current to within the rating of the micro steppers. You might need to change the values of the sense resistors on the board to get the current range to go low enough for your motors.
Another possibility is to run the boards at (say) 9V and connect a resistor in series with each stepper motor winding to reduce the voltage appearing across the winding to 6V.
Can you provide a link to the datasheet for the motors?
Edited 1 time(s). Last edit at 12/19/2014 04:32AM by dc42.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].