Thanks John for your feedback.
The very quick response from Anthem - Kudos for speed in which they replied to my email.
"They have changed the program without me noticing.
The only issue will be that it will look for a 7.1ch system if you enable the rears.
You may have to connect and disconnect the rear speaker. From ch 6 to 7 to have ARC work properly.
We will try to implement this 6.1ch feature into ARC."
If I am reading this properly at this stage they do not support 6.1 and so it seems best that I buy two more rears to go 7.1 and not 6.1 as planned. Going to look at bit messy... oh well
I am not very technical so maybe I am missing something
Sounds to me like he's saying there is a problem getting the ARC V3.0.1 setup app (the Windows program) to work with a 6.1 speaker system on the MRX because if you enable the rears when doing the ARC setup for the MRX it mistakenly assumes you must actually have a 7.1 speaker system.
Which means ARC will attempt to send test tones to both rear speakers and will, of course not get a response from one.
And then he's suggesting you can fool ARC, for now, by moving the output jack for your single rear speaker from the left rear output of the MRX to the right rear output and then telling ARC to retry that unresponsive speaker. You would have to redo this for each mic position.
ARC will then build a 7.1 speaker solution. Which I guess you then proceed to use with your actual 6.1 speaker setup.
I'm not sure there aren't still gotchas in this. I don't have an MRX so I can't be much help on this one.
Oh well, back to square one...