kenwood 718 has a good bass .. i think its the best availble speakers to produce bass ..anyways...
i suggest u to keep these speakers and HU installed in ur car.. buy a decent 4 channel amp and a reasonable woofer.. for low budget ppls kenwood 3009, 3010 woofer are good however pioneerD2 is better but required some more power..
after saving some more in some months change ur speakers and HU as well .. as 12K would be less for all these equipments...
as u r using 718 directly with your HU thats why u dont like its bass .. if u use these speakers with a decent amp u will love the bass of it (Y)