If you hae floorstanding speakers and you want to get a sub, when you set the speakers up you set the front speakers to small and enable the sub and it will run all the bass through the sub and not to the floorstanders (at least I think this is how it is supposed to work).
As far as the subs go, the Athena is probably a good choice and the Advent is a great good-for-your-money sub. However, it all depends on your ears. If you have heard the Athena and you think it will blend well with your setup then that is what I would go with. But, I would try and listen to some of the other subs if possible. Also, I would recommend looking at either the Cadence X-Sub or possible a Dayton sub. One more thing, keep an eye out on eBay for the Advent sub, there was recently an auction that was selling that same Advent for $35 which is a great bargain. Regardless of what you might think the Advent subs are very nice for what you are paying for them, they can easily stack up to most subs that cost $200-$300. Have fun with your purchase.