when your achieving spl from a car stereo system, its more 80% install and 20% equipment. i have seen plently single sub setups that have better output than multiple sub setups. my point is...the sub enclosure plays a VERY big role when your trying to achieve the best output from your subs.
what alpines do u have? if their the type-r's, then stick with those and throw them in a better box. (ported tuned)
snoop around
www.caraudio.com theirs plently info about car stereo their
if your on a budget for sub amps, check out hifonics brutus lineup
www.onlinecarstereo.com . eventhough their overrated in power, they still give a great value for your money. but id stick with the 500/1...if not, you can sell it to me =D