upgrade my sound!!

I personally have never used more than 1 sub.  I like your speakers and have owned an earlier iteration of them.  What is the goal to be achieved with the second sub?  Better bass for music or louder booms for action movies?

I'd say both.... people say another sub won't mean louder booms....rather a more balanced sound..... 

I recommend a Stereo Pair of Self-Powered Subs. Front Firing, No Ports, located near the Mains for excellent Imaging. 

Very low bass starts mono, then becomes directional by either the not too low fundamental and by the bass overtones.

Add 2 subs. Three will give you incredible smooth, tight, articulate and deep bass like you won't believe. Do not sell one, add two. Put each one a different distance from a corner.

Right now with one sub there are areas where the bass waves cancel creating a dip or drop-off. So you turn it up and this is what creates the boom. Running more subs in different locations moves these areas around until the overall response winds up being very smooth and even. Smooth bass is fast accurate bass. It will also go a lot deeper. Incredible improvement.