I have a JBL bass tube in the trunk, and I was wondering if there was any difference in sound acording to tha sub placement in the trunk. It has a port in the back of the tube. So if it does matter, wich way should my sub (or any sub) be facing?
As far as subs being omni-directional, it only occurs at lower frequencies. Once you get upward of 100 hz, the source of bass becomes more and more focused. I always recommend getting a solid midbass up front so you can allow your subs to be crossed over lower. That'll help the front soundstage a lot. Many cars sound a lot better with subs aiming into the car. It's the same as in a truck, a lot of times the rear seat will provide air pressure, and the seats won't rattle like a trunk.
60 hz is a pretty standard recommendation. I've had them around 80 without killing the front soundstage. There's no specific frequency, a lot of it depends on the car, the components that are used up front (and what they are crossed over at), and your ears. I was mainly pointing out that 100+ is when it gets really noticeable.