Exactly. How much fun would it be to say that your sub displaces more air than your car engine? lol Resonant Engineering is supposed to be making a sub that has 47 mm of Xmax. But that's for cars. I don't know how well it will work for HT. Car subs have to factor in the transfer function of the car in their design. We'll see though.

Xmax is great to look at. Lots of linear response means lots of fun. Especially with a big cone to have massive displacement. Efficiency is alright. The efficiency of the enclosure would be better than that of the driver. Is that what you're saying?