I have an 18" jbl in my studio, absolutely overkill for sitting in my chair mixing, but it works.
I usually only turn it on when my band is practicing, we have a lot of bass in out music between my electronics and a live bass player, so when we practice, we try and make it really loud like we are at a club, and tweak from there...I dont know if thats the best thing to do, but I've noticed we have been sounding a lot tighter and less muddy lately.