Tuned my all my motors last night and manually set the currents using Protuner. Much happier machine now, the X axis has gone from stalling at 8m/min to being able to do 15m/min without flinching. The Y can do 20m/min, but I've set both X and Y back down to 10m/min for general use.

Still getting some quiet hissing and squealing, although it seems to be emanating from the motor drivers not the motors themselves. Is this something I need to keep an eye on?