It is common pracice in pro studios to link all the earths together and ground through one earthing point (usually the desk)to remove earth hum problems and also to ensure that every unit is earthed correctly - by disconnecting each piece of equipment, or as you have done in your case with the dbx, you have isolated the problem - you may also find that a lot of signal to noise ratio can be minimalised by running every piece of equipment at +4db, i.e that every lead has an earth (balanced cable) rather than being simply 2core or monojack (unbalanced), running as a 3core wiring system greatly improves the electrical performance for not much bigger cost although if you're soldering the plugs yourself it can be a real pain. Also, there used to be a problem with certain equipment that was wired for live, which involved swapping the functions of pins 2 and 3 (I think) in 3 core cable so this could also be a problem. Hope this helps