The Denon looks to be the culprit. If I remove the 3 RCA cables running between it and the ADA, the hum disappears. I noticed that the Denon uses a 2-prong wall plug, so no ground there, but it does have a 'SIGNAL GND' connection point at the back, which the manual states is "used to connect a ground wire for the turntable". I tried connecting a wire to that ground connection, and then touching the other end to various points on the ADA, but that didn't eliminate the hum.

Axiom M5HP VP160 M3 ADA1000
Anthem MRX 720
SVS SB-3000 (dual)