In LA, I had a pair of 302's that had "anechoic chamber" style foam in the back to diffuse sound reflection and break up standing waves.
The 303's however, have a blow port in the back.
The port has golf-ball like dimples, which allow better airflow so you don't get stagnant air jammed in the same place.
These sound pretty good, I thought the 302's sounded great, more full, these sound slightly more open and relaxed.
I find B&W bookshelf speakers + NAD amps to be a very difficult combo to beat.. highly recommended.
