Interestingly, one person mentioned to me on my forum that the THREE and FOUR have an aluminum cone 6.5" midrange, so it's official, it's all aluminum, all the time
So, basically, a similar design to the more expensive Energy Veritas speaker. The cone probably rings around 4-5kHz, the dome probably around 12-15kHz, so they'd need to get that ringing at least 40dB down. Fortunately, the midbass crossover is at 800Hz, so that should get us down by 45-50dB. On the dome, they're crossover over at 3.5kHz so that would put us down to 36-48dB by 14kHz. I suspect the crossover points are specifically designed to push resonances down below audibility. The tweeter could be crossed over higher, but the dome probably can't. Likewise, the midbass could go higher but not if they want to remove all the ringing, so I'm pretty sure that they specifically chose the points/slopes to deal with it, just as Energy does with Veritas. I guess I'll just have to hear it. I hope they have some in a useful place to hear them.
Actually, the similarities between the Veritas design and the THREE/FOUR design is rather striking. The biggest difference is that the THREE/FOUR are both a little less than half the price. The FOUR also has a single 10" rather than dual 6.5" "Hyperdrive" bass drivers ("Prepare for Ludicrous Bass!!!") and the THREE has an acoustic suspension midbass instead of ported. Both have a narrower, lower diffraction baffle. It would make a nice comparison since the designs are so similar otherwise.
So, Tex-Amp has a point, but I have faith that NHT has dealt with the aluminum thing well, especially since they have previously expressed a high degree of senstivity to ringing. They couldn't do it nearly as well with a 2-way mid/tweeter combo, so the 3-way dome design is critical to avoiding this issue, which is why, I believe, the AZ and TWO use polypropylene. Well, I guess the proof is in the listening. Just kinda ironic though, huh