If Lars was evasive, he may have been avoiding a long, drawn-out discussion. I do not see any repeatable reason for concern at this point.
Not sure how you came to this conclusion.
Lars very much indicated the phenomenon was real and that the Purifi driver is particularly susceptible. For those that haven't went to the DIYaudio thread linked earlier here are his commments.
frame-motor resonance: this is a generic thing for all electrodynamic drivers. there is a heavy motor hanging in the frame which is bolted to a box. The frame acts as a (very stiff) spring and the motor is a mass thus forming a mass spring oscillator. All the Newton forces of the driver goes through this mass-spring system. This makes it very critical how the frame is fixed to the box since the high forces can cause rattling (ie high distortion).
It would be a lot better to mount the driver by its motor which is the source of the Newton forces but this is not standard. The best would be to bolt the motor onto a heavy solid chunk of metal serving as inertial reference to absorb the Newton forces.
Anyway, this is as mentioned a generic problem for all drivers but with the X stroke PTT drivers we have high Mms and a very strong motor in order to get low F3 in a compact box and this means the driver produces more Newtons force for the same SPL compared to a lower Mms driver. This makes the mounting method more critical including how much torque and what materialsused etc.
When measuring on the driver laying on its magnet the problem is gone since the Newton forces terminate into the 'ground'and the frame is untermintaed so we do not have the mass-spring oscillator.
I don't think the Purifi datasheet shows this behavior but we have the following examples all showing this behavior to some extent.
ASR Directiva R1 - ASR Review
Celuaris SPK5 - ASR Review
March Audio Sointuva - EAC Review (not as pronounced as others but still there)
March Audio Sointuva - @Nuyes measurements
To me all of this says the issues are real and if you want to avoid that 380 Hz resonance you need to think about how the Purifi driver is mounted / supported.
Michael