Speed control on AX500

I'm very interested in using your AX500 in a remote camera pan and tilt head application. I have only one requirement. Will this controller (using dc tacho feedback and controlled by a resistive joystick) run a motor very, very slowly at one end and also full speed at the other. Your data sheet seems to suggest it will. The reason I ask this question is that I've tried a few cheap board from Dimension Engineering and Pololu and there is a dead area where the motor has no speed, then suddenly jumps into life at about 200rpm. I need far more subtle control at the low end and less at the top end.

You need a controller that has drivers that will do Synchronous rectification. Another term to look for is "4 quadrants" controller.
The AX500 uses an inexpensive driver similar to the one you find in the other boards you used and has the same limitations.
You need to go with the AX1500.