But, in this particular case, the author went out of his way to report that this speaker was different in this respect.
Hi Joe,
The issue in question isn't uncommon:
Between the crossover point of 2.2kHz and 5kHz, the tweeter is transitioning across the angled chamfer, increasing the output off-axis. This seems like a good thing because the response is close to linear when sitting up to 30 degrees off axis. However, this has consequences in rooms without acoustic treatments.
This is a simple matter of a directivity mismatch, which is to say a 1" tweeter will have a very wide dispersion pattern at the bottom of its passband (i.e. where the wavelengths are large with respect to driver diameter), while a 5.25" midrange driver's dispersion will be narrowing at the top of its passband (i.e. where the wavelengths are getting smaller with respect to driver diameter).
There are a couple common ways around this:
1. As seen in NHT's Classic 4 tower and Classic 3 bookshelf, you can use a smaller midrange driver; in the case of the NHTs, they use a 2" dome midrange down to 800Hz, which ensures a wide dispersion pattern throughout the midrange.
2. Use a waveguide or horn to constrain the tweeter's dispersion, which you can see on a lot of Harman speakers (JBL, Infinity, Revel), KEF, etc.
XTZ also looks to have an interesting solution with their Cinema series, i.e. use an array of tightly spaced tweeters down to a lower XO point of 1.2kHz, with only one of them handling the band above 3kHz.