Or do you think the enclosure should "float"?
Hi Rusty, I assume that the enclosure is earthed properly. As far i could see, the protective earthing is tied to both heat sinks, the PCB and the enclosure.
The enclosure earthing can float - relative to the circuit ground. This is very common and often used to minimize ground / earth loops.
But, I am sure your enclosures do not float. You can check the enclosure-earthing and the circuit ground for continuity.
I believe a DMM will beep like crazy when you stick one probe to the amp's power socket earthing tab and the heatsink or some screws.
I can not imagine there could occur a problem with the earthing and the circuit ground. The wiring and the isolation also seems to be good.
The need for the compound comes where the "sub" heatsinks(which appear to be nothing more than some short pieces of aluminum angle stock) attach to the aluminum extrusions that form the amps enclosure and serve as the major heatsink....
Ah, now i got it right.
At this point, a good thermal compound is a very good idea. You're surely right. The better the contact between the surfaces - the better the cooling