Advent speakers (along with AR and KLH) really implemented the "acoustic suspension" principle, where the enclosed air provided the "spring" for the woofer. Hold one of those old drivers in your hand and see how the cone flops around when you move it. On one occasion I had reason to replace the woofer in a KLH12 system. The woofer free air resonance was individually measured and marked on the cone. 14 Hz. It's hard to find a woofer today at much less than 30Hz. You can still buy a speaker system with an "enclosed" cabinet, but I don't think any are truly "acoustic suspension". Some kind of hybrid I guess. The air spring is much more linear than any mechanical suspension, and leads to low distotion.
