I was back at the customers today. Interestingly it only appears to be two circuits (with dimmers) which create the interference, all the others are fine when turned on. A third circuit located upstairs which doesn't appear to be anywhere near the system also causes interference.
The speakers on the other two zones (3 zone system) appear to be free of any type of interference whatever state the lights are in.
The difference with the problematic zone is that the speakers are running off a power amplifier connected via the multiroom controllers pre-outs.
When moving the power amps speakers back to the multiroom controllers internal amplifiers, the interference appears to go away.
Should I therefore assume as suggested above that the problem is between the pre-stage and the power amp. Would earthing the multiroom controller and power amp together help?
At present the system is running fairly well without the additional power amp (without interference) although I would like to re-introduce the power amp as it gives a lot more control and power to the speakers.
