All I know is that when I had my Quad 63/Gradient Subwoofer combo (built for the Quads), it was the most seamless dynamic speaker/panel match I've ever heard, before or since.
The bass was almost as boxless as the Quad sound. Though, it also lacked some of the kick/oomph of non-dipole subs. But that aspect matched the same character of the Quads.