Taking a look at the interior shots of the ms600, it doesn't look like Meridian went nuts to block any sort of rf or other interference. A single twist between the power supply and components along with little to no shielding. Would be interesting to see if the application of some cheap rf/emi blocking tape/sheets would replicate your findings.

Primary: MD600, MS200 (SpeakerLink to AES), Trinnov ST-2, Ayre V6-xe, Salk SS8 3x LRC.
Secondary: MC600 (whole house).
Zone one: 568.2, G57, MS200, Spatial Hologram M2SE.
Zone 2-6 (Awaiting assignment).