Congrats on your UM Miracle+DX100 combo.
As to why I pair the DX100 with the Rx Mk3, it's cos I find that the Rx Mk3's amp is able to improve the treble extensions, and provide a deeper/cleaner bass presentation than the DX100's amp. Don't get me wrong, the DX100's amp itself is very good but the Rx Mk3 is further able to provide that clarity. As to why, I don't know and I'll have to guess. In my speculation, although both have strong powerful amps, I think the Rx Mk3 may be able to deliver that raw power more cleanly than the DX100 and as such a clearer sound.
I don't always use the Rx Mk3 with the DX100. When I'm walking around, I use only the DX100 and keep the Rx Mk3 in my satchel/messenger bag. But when I sitd own somewhere in a cafe, etc. then I use the Rx Mk3 with the DX100.