I do not know a way currently to do that, but I will try to find out something. I looked in the manuals for the RMS2020 system, and there I didn't see any statements at all about what the reciever units would draw. Nothing about that in the 2040's manual either if I am right.
I've noticed what you say about the higher efficiency at higher voltage and lower amperage, and it's clearly interesting. However, the manual states that the MXIR's maximum allowed voltage is 12v, so I don't think I would like to push it higher than that. If all of them would have been CXIR's it would be of interest to go up higher in voltage though.
Maybe I'll drop a mail to Audio Limited to hear what they have to say about this.
Will se if I can measure them, and then come back with some results.
/Anders