I have a subwoofer in my upright cabinet, and I can tell you that I hate it. Because the cabinet is so big, it resonates so loud that even when I turn it almost all the way down, it's too loud. I was using a computer 2.1 system, but I am about to replace it with a speaker that fits in the original cab's hole. (A car 6x9 for me). With some decent speakers, the cab itself will work like a subwoofer box and the bass frequencies will resonate. So you don't really need a subwoofer.
Sent from my Casio keyboard