Usually when using scales the motors also have encoder feedback for a dual FB system and these often hard to tune, also if the Glentek amps use velocity feedback in the form of tach F.B. this has now been replaced by Torque mode control amps where the tach is no longer used.
There is a an instructional video on the Galil site by Dr Jacob Tal about the different F.B. systems and also one on tuning the PID on dual F.B. scale/encoder systems
I am not all that familiar with the Ajax/Centroid system.
Al.