A basslink is a great option for just adding some low end. If you are not a bass-head and just want a little extra punch to the stereo, go for it.

With the rear speakers... the only way they will sound good is if the front and rear of the speaker are totally seperated... when the sound from the rear of the speaker meets the sound from the front of the speaker, some frequencies start to cancel each other out... making the speaker sound terrible.

The basslink is powerful enough to penetrate the rear seat, without the need for extra "ports" to let the sound into the car.

If you are really concerned about it, you may want to try something a little more powerful, like a seperate subwoofer and amp combination.