Try an ATC C4 sub woofer. The ATC's were originally designed to get the low distortion and exceptional quality sound of the Quad 57 midrange but at much much higher SPL's and without the distortion inherent in other conventional designs.
ATC sub woofers are of the type of design that you do not notice that they are there - very tight and fast.
ATC sub woofers are of the type of design that you do not notice that they are there - very tight and fast.