well ...
u have 30K budget .. and u have cng cultus ... remember to put a bass tube at back seat is a hassel ... u will be fedup soon ...
try to make a compact board at back side by which u can get a back view along with hassel free drive
if u r looking for a SQ... go for a compact sealed box with a woofer which has high X-MAX
ur alpine pre .. oes it has subwoofer option . two pre (RC) output at back .. if yes then u can go for a 4 channel amp with a woofer and ur speakers.. if not then u have to change ur pre otherwise u will get the sound quality bit lower then the pre which have subwoofer option .