Geddes is a proponent of minimizing diffractions [which are secondary sources of the original sound itself, and hence directly affect the frequency response curve]. He does that by:
1) choosing the right shape of waveguide [in his design, it is the oblate spheroidal type],
2) making sure that the internal aspect of the waveguide is very smooth,
3) making sure the coupling between the compression driver in the throat to the waveguide is smooth and that
4) that the coupling from the waveguide edge (or mouth) is smooth with respect to the front baffle.
He then minimizes diffraction on the baffle itself, and minimizes higher order modes as well by using his patented foam plug. Notice the alarmingly large round overs and such (which minimize baffle edge diffraction). This isn't by accident, and wasn't done for "looks."
In fact, he will further state that any furniture/stand/etc... that is near the main loudspeakers should not have rectangular edges etc... to minimize any reflections from them to the listener. That is something I have tried hard to do in my room, although it is difficult to do from an aesthetic standpoint.
There are other waveguide shapes out there that are not the oblate spheroidal type. Although they may reach the required amount of directivity control, in Geddes' opinion, they do have more diffraction as a result. As such they are not what Geddes would prefer to use. There are also horn shapes that not only increase the efficiency of the tweeter on the low end but also have some directivity control. However, the problem with many of them is the diffraction. It can be alarmingly high and can cause that 'horn honk' that many enthusiasts complain about. Subjectively, I find that these deleterious sonic consequences increases non linearly at higher spl. It doesn't sound twice as bad, it starts to sound ten times as bad!
I don't have much to say about smaller waveguides, although I do own (4) of Gedlee's Harper speakers, which serve as surround speakers in my theater. They have 8 inch OS waveguides. I also own the JBL LSR 305 in the family room which I am tickled pink with.