As another point, how should subwoofers be mounted? Should its enclosure just be put in the trunk, or should there be some port from the trunk to the cabin?

Depends on your car. Obviously, the closer a speaker is to your ears, the better. And the less "stuff" between the speakers and your ears, the better.

But in most cars, there just isn't any room for a subwoofer in the cabin, and you have to trunk mount it. I've had to do it that way in my Accord, and provided the subwoofer is tuned correctly and everything is balanced properly, it sounds very good.

Having the subwoofer in the trunk means that certain frequencies are muted, so you have to make sure your car's rear speakers can make up the difference. During normal road travel, I'm probably not hearing much out of the subwoofer above 200hz, so I have to make sure my rear speakers will extend down that low.

Having my sealed-box subwoofer helps, because the bass is tighter and less boomy. This helps reduce a natural boomy-ness that comes from having a trunk-mounted subwoofer.

Crossovers and EQ corrections take care of the rest.