The ifi micro idsd has two stereo dacs (4 channels) available to JRiver. What more could JRiver possibly need?
Here's the difficulty: based on the specs and the manual the ifi micro does not have 4 channels of line level analog output. Have a look at the pictures and diagram of the device (there's a picture of the back of the unit on their website linked above). It only has two line level analog outputs (right and left), and a stereo headphone output on the front. I have no idea why it advertises itself as having two stereo DACs (maybe one of them feeds the headphone output and the other the line? Or maybe it has something to do with interleaving to reduce noise as blgentry suggested?). But it very clearly does not have 4 line level analog outputs. Where would the .1 output for your subs come from physically?
It doesn't matter what chips are in the DAC if it doesn't actually have any extra outputs. What you are experiencing is a widely recognized problem with expensive DACs: the vast majority only support two channels of output. As Brian noted, you might be able to rig something up with the SPDIF output on the micro, but that's a digital output so you'd need a separate DAC to turn that into analog and at that point you're trending into a "swallowed a cat to catch the mouse" type tailspin. And there's no guarantee that the SPDIF output is separately assignable from the analog outs (it may just mirror the output which would make it useless for your purposes)
If you want a DAC that supports 2.1 output in JRiver, you'll need to find one that has as many analog outputs as you need. In this case that's
6 because of the way that JRiver handles 2.1. Focusrite, Steinberg, and Behringer all make models that will work, although I don't know if any of them support DSD natively. I'm not sure I know of a multichannel DAC that does, but I'm sure some folks on the forums might.