I remember selling these back in the 70s. Back then, a planar speaker wasn't quite what the regular joe-consumer was ready for - especially for the taller versions.
I remember their sound as being a bit ringing and sounded quite forward in its mid-range. The taller ones could be driven louder but not by much.
The other problem was that they needed to be returned to the factory for repair. Back then, about 50% of the speakers needed to be returned mainly due to the glue used to hold the voice coil to the polystyrene panels, which would come loose when heated.
Lastly, the pizeo tweeter seemed to not be well-matched to the panels, accounting for the forward midrange.
Mind you, this is almost 30 years ago. I would hope that these problems have been worked out since then. I wonder how BES gets them to market these days?
Cheers,
David