Roon’s DSP has always been capable of doing crossovers/multichannel output. We do not have a canned subwoofer section, but all of the building blocks are there for you to put together a low pass filter in the right place and route audio accordingly. You could also likely set up whatever you need using convolution + an appropriate text configuration if you need even more flexibility.
Push mode (or not) is a setting at the audio device level–has nothing to do with Qobuz.
I can’t speak for sure about how well JRiver’s driver works with Roon today. I’d recommend working out your DSP in Roon instead of cobbling together a fragile solution out of two players.