Ah. The higher the voltage, the lower the current the driver can take without overheating. You really ought to be using 12V not 20!
Those motors should operate reasonably well at ~1.4A. AstroEQ ideally would be run no more than 1.2A/phase, but you can push that to around 1.4A. The reason is the reverse polarity protection diode in the circuit starts to get hot at higher currents (there are 4 phases - 2 motors each with 2 coils), so the current through the diode is around 4 times higher - its not quite that simple as the driver boards convert voltage to current, so actually the input current will be lower than the coil current, hence you can push the limit.
To set the current limit, there is a small via on the driver boards near the IC (if you look on the back of the board there is a white ring drawn around it). If you measure the voltage between there and ground, whatever value you get, the current will be twice that. I.e. Ilimit = Vref * 2. So really you should adjust the potentiometer so that Vref is around 0.7V (no more than 0.72V).
If the current limit is set too low, the motors don't get enough power and so will skip steps at higher speeds. If the current is too high, the driver will overheat and either start skipping step or go into thermal shutdown.