The Denon is almost assuredly good enough unless you are sitting 15 feet or so from your speakers.
It doesn't hurt to get a Klipsch center speaker. Do you notice a different tone from the center speaker versus the others when watching a movie? Some people are sensitive to this. Others, not so much. Personally, it drives me crazy BUT I have to admit that many other people don't care or mind or notice -- and anyway a horizontal center tends to be placed at a different level than the main speakers and sound a little different, anyway.
That's not a bad sub. But it also doesn't dig down low as well as subs costing less from places like SVS and HSU. Depending on your room size, it may also be underpowered for proper home theater. How large is your room, including any areas it is open to?
I'd say that a subwoofer upgrade might be the most impressive way to upgrade your current gear.
The goal of two (ideally identical) subs is not about increasing output very much. (You can get a lot more output for the buck by getting one massive ported sub, rather than splitting the budget between two subs.). The goal of two subs is to get more even, consistent bass across multiple seats. It can take some effort (learning how to measure your room and gear with the free REW software) and extra gear (like a miniDSP and a measurement microphone like a UMIK1) to really dial in a dual subwoofer setup (unless your room is a closed rectangle in which case it can be very predictable and you can probably get good results with just some free tools on your phone). But for the ultimate home theater experience, dual sub can put you over the top in terms of quality and the experience.
*and all this talk of subs assumes you are already doing bass management properly and have designated all your speakers as 'small' in your AVR setup, and are sending all the bass below c. 80hz to the sub.
