Are you sure the motors are wired bipolar series, and not bipolar parallel? If wired parallel, they'd be rated at 3.5 amps.
If they are wired in series, then I'd lower the VREF to the recommended settings.
My motors are 2.3 amps and I have my VREF set to around 3.0, I think (It's been about 6 years since I set it up.
Mine is in the box with the PC, and I power mine with 5V from a HDD connector.