What does a subwoofer solve?

I'm going to be very skeptical that the M900u is shy of bass.  Are you sure you didn't change anything else?? Speaker placement for instance?

Subwoofers usually have their own amps to drive them, and they are easy to integrate into a system. My Paradigms, at least, have several pots in the back to home in on what you need. Buy something like a stereo test CD and let your ears do the walking/talking.

Subs are meant for low freq extension. They "solve" nothing on their own.

Subs will make your entire system run better. They will lower the distortion in your mains when using a highpass filter. They will give your system more dynamic range (output). They will most likely add bass extension depending on your speaker. Subs done right are always better than no subs. 
 

not to mention you can place the subs for bass while not effecting the soundstage negatively. I can’t recommend subs enough but and a big but is you need a highpass filter to sound right.