I don't know much about audio stuff. Heres what I would like to know. I went to audio shop and the guy told me that for subs to work properly, I'd have to get good amp and nice box. Now, are the box that the local shops sell good or are the atealth boxes that JL sell better quality? I'm thinking two W3 10" subs, would those fit the in the stealthbox? Xtant 121 amp powerful enough for the 10s? Thanks

The Stealthbox is designed for 8" subs only - I do not know if it could be modified to accept 10" drivers or not, but if you were going to take a pre-made box and then have it remade anyway I would have custom boxes made to begin with.
I personally will go with the 8" drivers. The larger the driver the more air it will move, so the larger the area (space) you have the larger the driver you need - the larger the driver you have the greater the watts needed to drive them.
My A4 has not arrived yet so I have not been able to modify it, but I am planning the sound system I will put in it. I will go with the Stealthbox as it will be the least intrusive sub set up and two 8" will fill the area.
The plus side is that the 8" drivers will be faster then equal quality 10" ones and require less wattage from an amp.
Hope this helps and good tunes are worth it!