As an answer to why to add a subwoofer to your system, try this article:

http://www.soundstage.com/maxdb/maxdb200101.htm

There are also many other similiar articles at this website:

www.ecoustics.com

To summarize very simply, there is more deep bass in your system than your "woofers" can provide. Now that you have a processor, your 990, that can provide some great bass management, it might be time to look into adding a subwoofer to your system. I think you might be very surprised in just how much one adds to even a great two-channel system.

Next question, do you have surround speakers? Setting them up with proper bass management can be the next thread.