There is an option to switch the BXT module ‘off’ in the ‘control’s’ menu, so it is straightforward to compare .
Technically bass is extended to 20Hz at -3dB at much higher spls, ( although I never found loudness an issue with the THREEs alone) the cardioid response is extended down to 20Hz and the speaker now becomes a line source, so some floor cancellations are cancelled.
In terms of sound, everything is so clear, even low bass , they are like a ‘super’ three a three on steroids there is just more of everything .
Not just louder but in terms of resolution even imaging , but at a price of course.
I think you would have to try them and decide whether they are worth it.
Personally I always wanted a pair of Beolab 90’s because they had that drama,scale, but the Kii/BXt has that they really are incredibly good, you know that already because you have a pair!
They are without doubt the finest sound I have heard here.
Keith