There are differences in the midwoofers, over and above how they interact with the circuit. The woofer in the Professional Series four π has a shorting ring, so it generates much lower distortion than the others. It is very clear and detailed. The Stage four π sounds really good too, with full bass and surprisingly clear, rich midrange. The Premium four π is a little darker sounding, more powerful. Each of these speakers is best in half-space, quarter-space or somewhere in between.