I bought myself this car used (nice Alpine HU) last year, & replaced the stock 'non-premium' Ford speakers with their 'premium' speakers. Soon thereafter, I replaced those with some Sony X-plod 6x8's. However I still cannot get more than p***poor bass response.

Here's the catch- they sound quite a bit better without the engine running. I know the exhaust drone naturally covers some of the bass, but I've heard better sound out of an 86' stock Escort.
Best Buy says I need an amp & sub since I won't get any quality bass out of 6x8's. I don't need my car to wake up the neighbors, but sucks when my wife out does mine with her stock speakers in her Matrix. Any tips?

I wondered if I had a wiring problem from the head unit.