Integrating a sub well (or 2) is a real pain in the butt. It is worth it in my opinion as it forces you to really calibrate your system and go through another round of listening / tweaking. Having actual full range (flat down to under 30 Hz at least) makes a big difference to me - surprising how much garbage there can be down there).
However, more bass extension will only exacerbate unevenness in a less than ideal listening room. So I suppose its get your room as even as possible (if its not already) with the current speakers, then consider a sub or 2, then go through tweaking the room again as you'll probably need more acoustic work to address the new octave or 2 of bass.
I'd want a sub with onboard adjustable filter points and phase adjustment built in.
What are the dimensions of your current room?