Let me take a stab at this. You say that you have disconnected the audio pair at each speaker leaving only the power and the noise persists. It's very possible that a noisy component in one of the speaker amps is putting noise on the common power wiring which in turn is picked up by the other speaker amps. The only way to find the one that is causing the problem is to disconnect each speaker completely first in groups then, when you have it narrowed down to a group, one by one.
And sorry Valcom, I know you make quality products that I have used many times but this problem illustrates one of the reasons why you should use a 70 volt system with this many speakers.
-Hal
