@sdl4, If I may, yes, bridged stereo amps typically don't handle lower impedances well. They are better suited for insensitive, high impedance speakers, where the extra power can be better appreciated, with the added benefit of better stereo separation, though typically with higher distortion levels.
Yes, the CS 3.5's eq makes greater demands on the amp. But.... this is mitigated by cleverly using the impedance peak that typically naturally occurs due to box resonance.
Thiel CS3.5 loudspeaker Measurements | Stereophile.com
Compare fig. 1 with fig 3. (you can ignore fig. 2 as those measurements would be specific to the testers individual room). Jim Thiel with his uncanny cleverness has the eq nearly identically overlap the impedance hump with the eq boost. This not only relieves the amp of power draw, but it also mitigates the draw from sucking power from the closely overlapping 1st order midrange. Converting a deviance from linearity into an advantage. All towards full range extended bass from a relatively small footprint, with superior time and phase coherence. Brilliant!
Full range Class D has not traditionally worked best with low impedance loads, Specialized Class D subwoofer amps are different. Though these Class D sub amps are reputed to be the most failure prone components in the field. Parts and repairs are not especially likely to be counted upon either. On the other hand there is a new generation of Class D amp modules just coming to market that hold special promise. I'm eagerly waiting for consumer reports on these new amplifiers.