Ryanz,
I always think two subs are better than one. Especially for Stereo / Music listening. If your current sub integrates well and isn't over loading the room, another one will be just fine. It will give you better stereo imaging of low notes.
I have two myself and love them. Granted, my subs don't go down to 15 or 28 hz, but it's better with them for sure!
T