Try also have to remember that low frequency sounds work a little differently to high frequency sounds.
Facing the speakers in a certain direction does very little to low frequency sound output, as the waves tend to hit your ear drum roughly the same no matter which direction they come from.
That is why you can hear the bass from cars passing by you from a long way off, as the low frequency waves travel through the air and other, more dense, materials much more easily.
Usually, the bass your speakers put out will be much less audible right outside your car than it is inside. Strangely enough, it will also be less audible right outside your car than it is a long way away from your car. Alot of the background noise you get with your engine and exhaust as well as your higher frequency speakers will disappear as you move away from your car, leaving only the lower frequency sounds.
I see it every day in my own house. My house is built on a solid concrete foundation which is on top of solid rock. The sub attached to my computer is set at a specific volume all the time, I don't change it. In my room, the bass level is quite comfortable to my ears, and fits in quite nicely with my mids and tweeters. In the lounge room, the other side of the house, the mids and tweeters are inaudible, but the bass frequency is heard at exactly the same volume as it is in my room. The lower frequency travels through the solid floor whereas the higher frequencies get lost much closer to the source.
Just remember this when listening to your stereo. Just because it's soft when you stand outside your car, does not mean it isn't carrying. It just means that the background noise is overpowering the lower frequencies where you are standing. Walk further away from your car and I'm sure you'll hear it more clearly.
