My speakers go down to 35 and I still added a sub, it made a big difference. As for set up properly, It's simple, when the sub works/sounds/blends seamlessly with your main speakers, it's as if you don't even have a sub. I myself got a JL Audio sub as it has DSP built in, took me less then 20 minutes to have the sub blend with the speakers.
I've heard nothing but good things about using two or more subs, but I was limited to one. Just one was well worth it.