I don’t know if it’s true in the case of ML’s ESL panels and their dynamic woofers, but the perception that the woofer in a hybrid speaker isn’t as fast as it’s panels is often more a case of the woofer not "stopping" when the panels do, rather than lagging behind them on the signal’s leading edge---it’s rise time. Dynamic woofers tend to "overshoot", the moving mass of their cones compelling them to keep moving once they have started, and not stopping when they get back to their undriven/"at rest" position, but travelling a little past it, and then finally back their center position. That phenomenon of driver performance is called settling time.
To cure that fault, Richard Vandersteen uses feed-forward in his subs---sending a pre-distorted signal to the woofers, the distortion a mirror image (out-of-phase) signal to counteract and compensate for the known imperfect behavior of his woofer drivers.
Peter Ding of Rythmik Audio used the more common servo-feedback to control the behavior of his sub woofers, though the servo is of a new, patented design. Did you know the performance characteristics and behavior of dynamic woofers, no matter the cost, changes as the materials in the woofer’s construction rise in temperature during use? The Rythmik servo system compensates for the change in voice coil temperature resulting from high excursion, one example of it’s benefits. The Rythmik subs are perceived to "stop on a dime". No boom, no bloat, no fat. One of the few subs perceived as being "fast" enough for use with planar speakers.
And then there is the GR Research/Rythmik Audio OB/Dipole Sub, the only one in the world to combine the benefits of servo-feedback design (one of Brian Ding’s areas of expertise) with those of open baffle dipole speakers (GR Research’s Danny Richie’s specialty). THE best sub for music reproduction currently available, and it operates up to 300Hz, so it can be used not only a sub, but also as a woofer.
