The lowest I have tried taking the PSU down to is 20volts.

I have been running the TB6560 board at 24volts fine unto my old psu gave up.

I would like to run how much voltage as possible to gain the extra speed out of the stepper motors.

I am slowly going to work my way upto 30volts.

I don't know how much, or if I will get any extra speed out of the stepper motors going from 24 to 30volts.