How rare I agree with Mazer! LOL! I can't speak for the x-sub, however I am very familiar with Athenas. Their speakers are well regarded by many, however their subs are generally considered to be underwhelming.
I would say go with the Onix over the Athena if those are the only two options. Assuming a 7' to 8' ceiling in your room, you won't need anything too huge, however an 8" is going to be hard pressed to get you very low with any authority. Have you thought about a PB-10 from SVS? There's one on the b-stock list now for $389.00. There's also the Hsu STF-2 for about $40 less. Pricier than the Onix, yes, but a little more horsepower. I just remember how crappy my old 8" sounded when I started cranking stuff up.
