You are correct that the DSP520 is the centre. IIRC one of the M6 speakers were made MASTER as it is connected in a star (rather than daisy chain) configuration which means (I think?) that one of the Left or Right speakers have to be Master.

Is my thinking correct here? If so, not sure that cabling is in place to support daisy chain so if DSP520 does need to be connected to the master bank of SpeakerLink to allow it to be a controller then may have a bit of a challenge!

P.S. It did seem possible for the IR to be enabled on a non-master DSP (the 520 in this case).

MC600