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?
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?