Since JRiver Media Center essentially sends bit-perfect (data from music files) to PS Audio DirectStream DAC- The MC21 upgrade should have zero impact sonically.
The improvements in MC21 should be (meaning- we don’t know what they are changing/modifying) how it manages your library, and how it alters the original bits if you go that route.
As to the MC20 upgrade, many of us will pony up the money just to keep using the latest and greatest software for managing the music library.
side notes -
The JRemote is a separately purchased software/program/app and thus is not part of the MC21 licensing. JRemote allows the user to access the MC library remotely (say on an ipad). I find this program extremely useful to not have to go into a windows interface.
The MC20 is a piece of software. Firmware is usually software stored into a “device” - typical of Yale, PP, etc.
I run MC20 on an Intel NUC box with a Seagate 4TB portable drive attached for music files- and only output via USB.
The NUC Box also plays audio via HDMI (things like movies, youtube content, etc) into the TV, which feeds the Directstream using Toslink. Both USB and Toslink sounds fantastic. I like this setup because the NUC box only eats up about 10 watts and is silent except for the spinning external portable drive.