Off-axis they are a bit less smooth than a normal dome.
See:
This Test Bench focuses on SB Acoustics’ Satori high-end line of drivers, the Satori TW29RN.
audioxpress.com
vs
Test Bench of SB Acoustics' new high-end Satori line neodymium 29 mm beryllium dome tweeter, the TW29BN.
audioxpress.com
Also note the lower distortion on the BN.
Overall though, they are both quite good.
I thought off-axis was the whole point of the dimple:
1. Shape and Wavefront Control: The dimple changes the shape of the tweeter’s diaphragm, affecting how it moves and how sound waves are produced. This alteration can lead to a more uniform wavefront as the sound leaves the tweeter, which helps in spreading the sound more evenly across different directions. This is particularly valuable in a listening environment where wide dispersion ensures that the sound quality remains consistent over a larger area.
2. Phase Coherence: The dimple can also aid in controlling the phase of sound waves emitted by the tweeter. By managing how these waves interact, it can reduce phase anomalies that typically lead to sound distortion or uneven sound fields. This results in a clearer and more coherent sound across a wider listening area.
3. Directivity Control: High frequencies are more directional than lower frequencies, which can lead to a narrow “sweet spot” for optimal listening. The altered geometry of a dimpled tweeter can help in expanding this sweet spot by modifying the directivity of these high frequencies.
While I see the differences with the beryllium tweeter, it is more than twice the price - does the improvement warrant the price increase for this build? To be honest the 2-way approach is already way more expensive than the full range, so not sure I am willing to put down that much extra. I see that the AMT is also way more expensive than my original pick, something I did not factor in or check, when I added it to my notes.
Troels Gravesen also measured the tw29rn:
http://www.troelsgravesen.dk/SBA-tweeters.htm