Ok, so the X-axis chip on my grblshield stopped responding to direction input and I looked into buying a replacement. According the synthetos site and the markings on the chip it's a DRV8811.
Synthetos claims 2.5A per winding for the chip, but the TI site claims 1.9 (http://www.ti.com/product/drv8811). Its big brother, the 8818 IS 2.5A however (http://www.ti.com/product/drv8818).
So have I made a mistake somewhere or is the data about the grblshield incorrect?