Yes, I did try it and as far as I can tell it does not work.
I use it daily now, in fact. All my sound is routed into MC using its ASIO driver. My default Windows audio driver is "Hi-Fi cable" from VB-Audio (
http://vb-audio.pagesperso-orange.fr ). It has the capability ("ASIO Bridge") to route all sound to an ASIO device, for which I choose JRiver MC19.
The setup looks like this:
Windows sound --> Hi-Fi cable wdm driver --> ASIO Bridge (8ch) --> JRiver MC19 ASIO driver --> MC audio engine --> ASIO output device driver --> DACs --> amps --> speakers
It works as expected; I can use all of MC's audio engine. Only trouble is that the channel count coming in to MC is always 8. If only 2 channels (or 6, or anything other than 8 ) have any sound in them the remaining channels are silent. JRSS doesn't seem to detect this and kick in with its up mixing.
I could get JRSS to kick in if I changed the number of speakers in Windows audio configuration to 2. With Wasapi loopback this channel count was then kept all the way into JRiver. With the above ASIO method this is not the case though, the channel count going into JRiver is always 8. Regardless, I would not want to change Windows speaker configuration every time the source number of channels changes.
Edit: Could it be that JRiver only detects 2ch sound in 6ch container, not in 8ch container?