DTS-ES-Matrix probably doesn't get decoded as discrete channels (since they're not really).
Indeed, such is the nature of the format
You can look at Audio Path while playing to see what format data is being input.
Thanks Matt, I had already checked this and it shows the source material as 6 channels only, hence my original question.
So for anyone else reading this with the same issue, to date JRiver can not decode DTS-ES-Matrix, but it will happily Bitstream it.
The obvious solution here would be to just bitstream and have my processor decode the format, but this leads me to another issue...
My Processor can only decode standard DD & DTS and not Dolby TrueHD & DTS-HD-MA (although it can accept HD LPCM over HDMI) but seeing as the bitstreaming options for MC are somewhat limited either way I have to accept a compromise:
either send everything as LPCM and do without the back channels in Standard matrixed formats OR bitstream everything and only get the 'core' of
Dolby True HD/DTS-HD-MA.
JRiver could, of course, expand the bitstreaming options (as I have requested on the MC18 thread) and give users the option to choose what they want, I guess it would look something like this and my mock-up of the required options will probably make more sense than me trying to explain it:
(excuse my terrible PS skills)