Subwoofer advice please

The 57 worked stacked because it had a single tweeter panel with a bass panel to either side. Stacking turned a pair into a quasi-line source speaker.

The 2805, on the other hand, is essentially a coaxial design, the sound emanating from the center of the driver outward (via delay lines in the driver). Placing one 2805 above another will result in lobing and comb filtering. Not good.

Right you are, willem, it was indeed Gradient. DSPeaker and Gradient are both Finnish companies, and there is some relationship between them, though I don’t recall exactly what it is.

The GR Research OB/Dipole Sub is offered as a kit, the H-frames into which it is installed needing to be made (or have made) by the user. So shipping shouldn't be too bad. The dimensions of the H-frame are about 16" wide, 14" deep, and 28" tall, not too bad.

willem, it is very true that part of the reason most subs don’t mesh well with ESL’s and other planar speakers is the dipole nature of their radiation pattern, which excites fewer room modes than non-dipole speakers and subs. Their null to each side prevents the side wall-to-sidewall mode from being energized.

It is for that reason that dipole subs are embraced by a small segment of the dipole speaker fraternity, and why there are companies offering dipole subs to that group. DSPeaker themselves developed and manufactured dipole subs for both the Quad 63, and the original ("57").

But the really interesting and high-performance dipole sub is the one co-designed by Danny Richie of GR Research and Brian Ding of Rythmik Audio. It consists of two 12" woofers per sub, mounted on an open baffle H-frame, along with a plate amp containing a shelving circuit to compensate for the inherent dipole-cancellation roll-off as frequency descends. If interested, check it out on both company’s websites, and read discussions about it on the GR Research AudioCircle Forum.