I haven't yet been able to get a decent amount of bass out of regular speakers, even 6x9's. I've got a set of Pioneer 6x9's that will go down that low, just not with a flat frequency response. The lower you go in their "range", the less volume you get. You can only push so much bass at so much volume through them before they distort.

I do not use a subwoofer to "boom" down the street. In fact, the stereo sounds perfectly normal outside my car, no excessive boomy-ness.

The thing a subwoofer gives me isn't "boom". It's a proper flattening out of the low frequency response, as well as the ability to bring the low frequencies up so that they can be heard above the road noise.