ok i didn't read anything anyone else posted for 1... but imo NEVER buy a prefab box.. always build your own. (conveniently i have an alero and design boxes
for free) if you're looking for cheaper subs that are still loud and sound good I advise going with AudioQue (now sound qubed) SDC series
audioque.com
for amps, Hifonics are GREAT amps for their price.
for a head unit I like to stick to my higher priced pioneers or any alpine honestly. alpines my brand when it comes to head units. for speakers i like alpine too. the type-s series sound nice and clean powered by a headunit. with the front doors i would get the 5 1/4s and do some customizing to the stock front door speaker mounting pods (that's what i did anyways)
i have designed numerous boxes for my alero. let me know what subs you go with and I can design you a space efficient and nice box. I can tune the box to your preference.
That goes for anyone else on here who needs a box designed too. just message me and i'll get back too you