A small segment in the intro of Stereophile's ampeggio review as well as the measurements partly explains Voxativ's design approach. Maybe you have already read it.
https://www.stereophile.com/content/voxativ-ampeggio-loudspeakerCan I ask where you auditioned/purchased them? I have only heard the 9.87 a few times at shows and the Zeth wood-driver with Zbass module once. They are speakers that have made me stop thinking and just relax, despite their obvious limitations.
Voxaxtiv is active in stating they make woofers and not subwoofers. I appreciate their approach as the 9.87 system was I think the best bass I have ever heard. Perfectly damped, and like few others they avoid the desire for the dreaded bass "slam". Integration with the Pi was fantastic. I was not so much in to the Zeth w/ Zbass that day. Sound was anemic, although that was just single demo. At a 9.87 demo Voxativ employee said to me he prefers the clarity/detail and bass of the Zeth over the Pi. This confused at the time because the price line up is a little slippery. The customer has think a little bit about what each model is all about. Voxativ, though, has been marketing really well.