I have heard a home theatre system with the Sonettos and Gravis subs and it was exception given the price point. I would be surprised if the subs didn't provide you what you are looking for.
I personally would be more likely to upgrade the amp section than add a sub based on my bias that a 2 channel system isn't perfect and that's part of the experience. And some of my friends consider that approach as wrong as a football bat.
Let us know your decision...