Didn’t think there was ANY SPEAKER harder to drive than the 9’s.... WOW!
I know the Scintillas were close but DANG!
You can add the OP’s Thiel CS5 also CS7, Wilson Alexia to that list also and there are many others.
And to me also the "good" ridiculously hard to drive speakers are more extended, tighter more detailed in their presentation, this is usually because of the drivers used are inefficient more so the very complex xovers that are designed to be nearer perfect for these types of speakers create the low impedance loads .
There’s no free lunch, easy to drive, simple xovers, efficient drivers, even the use of horns usually have compromises, the above difficult to drive speakers some may say are over engineered, so is a Buggati Veyron ect ect.
Cheers George