Reminds me of calling a bug a feature. Good modern large compression driver horns like the one of the M2, some Limmer etc. show how such geometries have evolved.
The waveguide on the Nathan is inherently compromised by its significantly sub-optimal size.
The on-axis "hole" is something that happens with an axisymmetric horn or waveguide when the mouth reflection arrives in-phase but about 1/2 wavelength behind the unreflected sound. It can be mitigated by a sufficiently large round-over at the mouth, which correspondingly increases enclosure size and vertical interdriver spacing.
Non-axisymmetric waveguides like the one in the JBL M2 and the SEOS waveguides do not have this issue because the mouth geometry smears out the mouth reflection in time.
The recommended setup for Earl's constant-directivity waveguide speakers, and for mine (most of which use a non-axisymmetric device), is to use aggressive toe-in such that the speaker axes criss-cross in front of the listening area, and I can explain the reasoning if you would like. Let me emphasize that the same setup geometry is recommended for my non-axisymmetric waveguide speakers. In other words the toe-in is not a "fix" for the on-axis response hole, but it does make it essentially irrelevant.
One setting where the on-axis hole would be a major issue is in a recording studio setting, where mixing (and probably mastering) are done on-axis. Dutch & Dutch evidently uses a waveguide geometry which mitigates this issue.
I have built loudspeakers which are virtually identical except for one using a SEOS waveguide and the other an Oblate Spheroid. Ime the Oblate Spheroid is the superior device regardless of listening axis, but especially so when used as intended.
It is easy to lose sight of the forest (the desired outcome) for the trees (the measurements). The desired outcome is that the direct sound be as correct as possible, and that the reverberant sound be as correct as possible. The measurements are highly useful but they are not the desired outcome in and of themselves. In my opinion.
Also, please note that the spin-o-rama measurements of a non-axisymmetric device inherently MISS what is happening along the diagonals [NO they don't; I was mistaken - see Ilkless's
post below], and just as much sound is delivered along the diagonals as along the standard measurement axes. Eyeball the JBL M2's waveguide and you will see what I mean.