Amps

Your 809 has preamp out jacks - RCA type. You connect the XPA-3 to the Onkyo via those jacks using the front left right and center channels. You'll also need to move the speaker wires from the Onkyo and connect them to the XPA-3. The XPA-3 would more than double the power to the front three speakers. Whether you need an external amp or not would depend on the speakers you now have - the size of your room - and how loud you normally listen to movies and/or music.

Its kind of like adding a supercharger to a car - granny ain't likely to notice but her grandkids will.

The "Amplifier FAQ" (sticky thread) is excellent reading to learn about amplifiers. Will your system improve? How? In what way? Assuming you use the XPA-3 to drive your LF, Ctr, FR speakers I think you will notice a significant difference. That said, a $600 investment towards amplification may not give you the biggest bang for the buck. Have you thought about upgrading your speakers? As a generalization, you will notice more of an impact for your $ at this point. Hope this helps. . .

For$600 id recommend just getting another sub. Id only get an amp if you want to listen louder. That's just me though. Check out the amp faq

What sort of improvement are you looking for ?
If you provide room dimensions this will help.

Man you really have the upgrade itch ahah

For your situation, I'd second this^^. Your room is not that big, you have a very capable receiver and very good speakers that are fairly efficient. Adding an amp, wouldn't give you an improvement on sound but rather more headroom.

I think adding a second sub or treating your room acoustically (if not already done) would yield you more immediate noticeable results. Just a thought.

cheers