Yes more subs will be an improvement. But not "by taking full responsibility for the very bottom range." The best way to do it is to continue to run the stereo pair full range, and add subs. That's what I did. https://systems.audiogon.com/systems/8367 It works great.
Run the subs off the RCA. You can run 4 no problem.
Run the subs off the RCA. You can run 4 no problem.