..."adding a sub" ........
is something I have done for YEARS to my self powered kbs. In many cases, the volume of the on board sound system is "almost" enough, but just missing the extra "thump" that the bass requires. Adding a small bass amp, or a powered sub is as easy as pluging in a wire from the kb output to the amp input.
I play at least 30-40 jobs each year in very small, intimate locations and this is the system of choice for those situations.
If you're just doing background music ..... there is no need to bring in speakers that are more appropriate for a dance. Size counts, and appearance too.

I played a house party last Saturday in a 2 million dollar home, and any amps at all would have been an eyesore. I uses one small bass amp on the floor UNDER my kb stand and that filled the house with all the sound I needed.