Also consider that the Delta 15 has a thin light cone which means the cone is flexible at higher frequencies, so the total mass and size doesn't prevent the higher frequencies. With strong paper and at the associated moderate power levels this is usually an asset as I believe is the case here. This is why they don't shut down at 1000Hz like the Magnum or Kilomax. As for the HF driver its 2" once you get down into the guts. Horns load that light diapragm to make it "feel" bigger and heavier, but up high the throat is to big to resist that huge honkin' magnet, yet small enough not to cancel out the top octave. So you still get enough top end. Most bigger drivers have no good top end.
Enjoy 'em!
Thomas