So no one can explain what changes the above setting makes?
I think you should set it to 2.0 seeing as though your DAC is 2.0 only.
Setting it to 2.1 in JRiver means that JRiver may filter out frequencies below the Subwoofer setting in DSP Studio and attempt to add them to a .1 channel.
I don't think you really have a 2.1 system, you have a 2.0 system because that's what your DAC is.
Your subwoofer is not a true .1 channel receiver in your setup because there's no way for your DAC to exclusively pass .1 to it.
You simply need JRiver to down mix 5.1/6.1/7.1 movie tracks to 2.0 which it will do if you set Channels to
2 channels (stereo) and Mixing to
JRSS mixing (recommended).
Also set Subwoofer to
JRSS Subwoofer (120Hz low pass) or thereabouts and this way yucky nasty .1 LFE above that frequency garbage on poorly mastered bluray movies such as the original Star Wars trilogy won't be a problem.