four 15'' subs

Other than dependibilty, I can't say you could find a better SPL sub than the L7 for the price. They're a top dawg SPL sub. BTW what kind of  space are u workin with? 4 15's wont fit in many setups.

I would put my money on 2 of these or 2 directed audio, these will be better than 4 cheap ones

http://www.cardomain.com/item/ROCP315D2

http://www.bizrate.com/marketplace/product_info/overview/index__cat_id--11940000,prod_id--131783643.html

http://www.bizrate.com/marketplace/product_info/overview/index__cat_id--11940000,prod_id--131783639.html

these 2 memphis m3 http:///.com/dll?ViewItem&category=18803&item=5764045859&rd=1

these 2 ma audio http:///.com/dll?ViewItem&category=18804&item=5764932489&rd=1

but if you need 4 for 150 per check these out

http://www.bizrate.com/marketplace/product_info/overview/index__cat_id--11940000,prod_id--7390384.html

http://www.bizrate.com/marketplace/product_info/overview/index__cat_id--11940000,prod_id--7580046.html

http:///.com/dll?ViewItem&category=18804&item=5765611772&rd=1&ssPageName=WDVW

http:///.com/dll?ViewItem&category=18804&item=5764216067&rd=1

or do a search on ebay and pick some out