07-20-08: MusicnoiseProblem is if a piece of equipment introduces a ever so slight ground fault current in the signal ground to equipment ground I believe the ground loop hum will still be present and will still travel through the ics.
Connect all chasses together, so that you have one electrical point, use large guage wire. Now use one path to earth from the chassis - i.e. one wire. This should eliminate a ground loop problem.
With your added chassis ground the ics are still running parallel with it.
The offending piece of equipment or ic needs to be identified and corrected....... Jmho.