Everyone, thanks for the replies! What was posted was helpful, though it does raise some questions.
JimH, I'll note that the variety of answers on what output format to use underscores my contention that lack of documentation is a pervasive problem.
What I'm trying to do is take the bass from each channel and route it to a corresponding sub by high-passing the main speakers and low passing the subs. That is what is people usually mean by a 2.2 system. It's just like a 2.1 system, except you keep the R and L bass separate. It should not be complicated!
So yes, I want to output 4 channels, which can be done through the Hilo ASIO output. I don't think "2 channels in a 5.1 container" would work. If that really does output > 2 channels, the JRiver people should change their terminology.
Also, I don't think I want JRSS Mixing. I asked about it years ago and was told (essentially) that it is a proprietary algorithm not explained to users. Not what I'm looking for!
So, guys, I do appreciate the answers, and if I pick among them, I just might get a working 2.2 system.