How should I power these stepper motors I am rebuilding my printer, a hold Prusa/Mendel that has been boxed for years. I have changed the board ( I am using a Rumba) and the drivers (I am using Pololu 8225) with heatsinks and 1/16 microsteps.

5 motors type 42BYGHW811, rated: 2,5 A, R 1,25 Ohm.
I have set the stepper voltage at 0,8 V for XYE which gives a current of 0,64 A and they seems to work without problems.

To get the same current on Z, where I have 2 motors in parallel, I should set it at 0,4 V but the motors do not turn at such low voltage. They just make noise, no turn. I have M8 rods nicely lubricated on such axis and I can turn them easily by fingers. If I go up to 1 V the motors turns but then the driver gets very hot and I loose steps. May be I should play with the acceleration too?

I am not sure if the above settings are correct. Your hints from your actual settings and experiences will be appreciated.
If the motors are in parallel, then setting it at 0.4V means each motor will only get a quarter of the current a single motor would get at 0.8V. If you set it to 0.8V each motor will get half. I think 0.8V (same as for the other motors) would be an appropriate setting. Stepper drivers are constant-current, not constant-voltage.

Are the steps/mm for your Z-axis correct? Perhaps it's just trying to move too quickly.