I've heard the Kii Three without BXT and I was quite impressed with the sound. My understanding is this- the BXT essentially let you play louder without the speaker protection kicking in (as well as offering some additional extension, more on this later), from what I am reading Kii have made this level fairly high with a firmware update that came out a while back. I can't recall what the SPLs were off the top of my head but I seem to recall thinking it was at the limit where no one would realistically want to listen at those levels, and I do listen to primarily classical so lots of music with wide dynamic range.
My reasoning was either way for my average size room that I intend them for I am going to need to do something to smooth out the bass. The BXT does not fix this issue, it will extend the bass essentially turns it into a truly full range speaker. This does nothing for the room modes and where I think Geddes or
Welti multi-sub is still the most high fidelity way to do it. So either way I'll be buying high quality subwoofers to fix these issues, these will also offer the additional extension and relieve some of the low bass duty that would presumably normally kick in the protection without the BXT. Bringing me back to my point that at least for my use (anyone's use if they want truly flat bass?) I can't see the advantage of the BXT.