Tough question to answer!

I have heard, but have never tried, that if you have two subs they should be identical models.

I think what it comes down to is ask yourself
"What I am lacking?"
Is it more bass or more surround ambient sound? If you have flat spots in your bass response, I'd say pick up another sub. If you find the ambient sounds are getting lost in the back, get more surrounds.

But remember, if you go to 7.1 you may have to upgrade your receiver and not DVD's are actually mixed in 7.1