There is a simple way to test to see if it is a ground loop. You can get one of those cheap plugs that convert a 3 prong to 2 prong. That eliminates the ground to the equipment.
If the equipment is on a power bar or conditioner, you can use that 3-2 plug on that, and the equipment rack will be ungrounded. If you try that, and the hum goes away, then you need to figure out which components are contributing to the problem.
I am not recommending that the plug be the ultimate solution to your problem, or that you run your rack long term that way. It is a very cheap, fast way of eliminating ground loops as the source of the hum. Ground loops are very real, and often hard to resolve, even with 'good' equipment.
It might be a very sensible thing to try before you rip into your equipment. If it is a ground loop problem, it will likely still be there when you are done rebuilding your equipment.