Comes down to budget.
Pioneer 307C 12"s are $118 each and nothing will beat the output of them for even double the price of them. Pair of them with 1100wrms and you will be happy. I had an Alpine PDX1.1000 with a pair of those and it was decent in a Kia hatch. I installed a PDX1.1000 with a pair of Kicker L7's in a mates car and it didn't pump much more (Kickers are $500++ each), he just upgraded to a pair of Type R 12's ($250ea) and that hammers =)
But trust me that is enough bass until you get the front speakers and head unit high quality. Then focus on sub quality afterwards. Otherwise go all out..... what is your budget?
1989 VN 5 Speed – Turbo conversion
1997 BMW E39 M52B25 Powered.
1994 Yamaha XT225 Modified