As someone who designed several OEM car 6x9 subwoofers when at Goodmans, I can assure you they bring an interesting set of design challenges compared to round speakers! That said, I must admit I'm surprised there hasn't been more of an effort to come up with something like a modern KEF B139, which would bring the advantages of a decent cone area, but still in a width that would fit a modern, slim cabinet.

As an aside, one of my good friends and former colleagues worked at Goodmans for several years before I did, including the time when Mordaunt Short were in the same building and developing the 800 series mentioned above (which had the elliptical drivers).

They had 'one or two issues' with the drivers' design and my friend was loaned to them for several months to sort it all out, which he successfully did, and rumour has it the 800s are rather good. However, to this day, he still develops an inadvertent twitch if I mention them in his presence!