I don't know what case you're talking about. Also keep in mind that Asus Xonar is a huge card, and won't fit in most small cases.I was thinking about putting it in the double-din black case if it would fit... would like the computer out of the way.
One doesn't usually buy tweeters separately, since that takes expertise in picking ones that sound good with the woofers, and getting an appropriate crossover. If you already know that this combo will sound good together, then great. Otherwise, you probably want the JL Audio C5-650 (without the "x"), as that is a component speaker set with separate woofer, tweeter, and crossover.Speakers
Tweeters: JL Audio ZR100-CT
Doors: JL Audio C5-650x
Hope this helps!