My bias against powered bass in high efficiecy speakers is just a thought that since these designs use transistor amps for ther woofers, are they forgoing the supposedly sonic superiority of SET over SS amps (even in the bass quality.)?
I don't know how you would actually like it, but before you jump to conclusions about it you should perhaps try listening to some implementations of what you are describing here (basically you're talking about bi-amping). I do think it takes some skill and knowledge, but have heard it done well, though I cannot say it was with a SET amp in the mix. As far as the sonic 'superiority' of SET over SS - IMO (this is coming from a big SET fan who has used 300B 8 watt SET amps in my main system for around 8 or 9 years now) the allure of SET is all in the midrange purity and that magical holography that no other type of amp does so well (OTL is a nice alternative though and comes pretty damn close). I haven't heard any SET amps that can do tight convincing bass as well as an SS amp - that's not to say one doesn't exist, but I haven't heard it if it does. When it comes to pushing those larger woofers that really move a lot of air fast to create the convincing bass, the lower powered amps just are not up to the task when compared to more powerful alternatives. I'd also highly recommend SET and horns, and would have mentioned vintage Klipsch as well but your tastes seem to run in the more exotic realms. Klipschhorns and SET were made for each other. Horns and SET in general are a great match, but they can also be a matter of taste as the sound is unique. I love it myself, but have also heard front loaded horns that bore me to tears or make me want to go running from the room. I tried Zu Druids for a while and didn't warm up to them at all. The success of your implementation of SET and a particular pair of speakers will also be determined by the kind of music you listen to, how loud you like to listen, and the volume of the room you are listening in. Most of the music I like the best is well suited to SET - it is simple, acoustic and or vocals in small arrangements. If you are into densely arranged orchestral, jazz or rock music IMO there are better choices in amplification than SET, and the idea of 'superiority' goes right out the window in comparison with an excellent SS amp when listening to these materials regularly. In this sense SET is kind of a one-trick-pony. It is certainly not the Swiss army knife of amplification. Don't get me wrong, I love what it does well - in those realms it is king when put together with the right speakers. You are right to carefully research that specific interface as it is more critical than ever when using a SET amplifier.
Good luck!