Tbg,
I know of speakers more expensive than Zu Definitions that are different and might be better in some environments. However, one thing I am learning about Zu speakers is that all the models are so good at their price that they shift the focus to the quality of the power amps. In fact, rather than spend any more on speakers, I think it is entirely sensible to end up with power amplification that is considerably more expensive than the Zu speakers are. I have both Druids and Definitions and on both I have amps considerably more expensive than the speakers. I have no doubt that in the case of Zu Defitions, it is better to put $9,000 in speakers and, say, $16,000 in qualified amps, than $20,000 on other speakers + $5,000 on amps. This principle applies to the Druid even more dramatically.
You cannot properly audition and evaluate Definitions unless you are in a room that allows the listener to sit AT LEAST 10' from the face of the speakers. Any closer and the tonal balance is disturbed, octave-to-octave tone no longer consistent. The Druid works wonderfully closer than 10' but the Defintion will not fully integrate closer. This was the foremost problem with Definitions at RMAF and yet many people heard through that.
I have heard Lowther-based horns and other architectures, and have not liked them. Very good dynamics, small windows of holography and strobe-like flashes of uncanny midrange tone but overall not natural, consistent or convincing to me. The Zu driver as implemented by Zu has broader truthful frequency range, more speed and impact where appropriate, is considerably more tonally accurate to me, and you get a more practical sweet spot. But look, I've never heard a Lowther-based speaker that to my brain has not been crippled by deeply distracting tonal anomalies. The Zu driver is convincing whereas Lowther, Fostex, etc., have not proven to be so, IME. Many other people obviously are hearing them differently.
Someone local to me who was at RMAF opined that the Bastani Apollo is better than the Definition, $15,000. I can't say as I haven't heard them, but....Dude can hear, so maybe he's right.
Phil