[Folsom]

My suggestion was to ground the shielding of crossover box to the RCA grounds.  I did not say a safety ground. 

As also suggested, the RCA cables should be well shielded to reduce the RFI if that is the cause.

The best way is to isolate the RFI source and disable it.

Sorry, people usually mean safety ground when they just say ground but don't specify it within a return path.

[Speedskater]

It's so confusing, 'ground' has so many meanings.
Just some of it's meanings:

Safety Ground/Protective Earth (Equipment Grounding Conductor)
Mother Earth (Grounding Electrode Conductor)
shield (chassis)
audio circuit common
signal common
DC supply common
equipotential point
While most of these are galvanically connected, each has a different task.

Or we could use the Henry Ott definition:

Ground - A path for current flow

[tasar]

Curious what MLundy finds. I've run separate hard wired feed from main box to the audio room only. One single cryoed (plugged both grounded and ungrounded) line to a UberBuss feeding all components, to no avail. Two of Danny's filters lay waiting. I can insert one before a monoblock with no noise, insert the second, and hum returns. I'm beginning to think as most studios apparently, an entirely separate outside ground rod is suggested.

[Folsom]

Curious what MLundy finds. I've run separate hard wired feed from main box to the audio room only. One single cryoed (plugged both grounded and ungrounded) line to a UberBuss feeding all components, to no avail. Two of Danny's filters lay waiting. I can insert one before a monoblock with no noise, insert the second, and hum returns. I'm beginning to think as most studios apparently, an entirely separate outside ground rod is suggested.

tasar that really isn't enough information to understand what's going on.

Sure, studios do that. But they also have a special mixture they put the rod in for much lower impedance.