The new Zu Definition 4 uses a Radian 850 compression supertweeter, and it is a major improvement over the prior Zu supertweeter. It may be the most beautiful supertweeter I've heard in any speaker of any kind at any price. Now, in Def4, that tweeter is only rolled in on a high pass filter above 12kHz, so what's going on up to 12kHz? Well, there things are improved for resolution and listenability too. The new nano-treated Zu FRD is even faster, more agile and more resolving than the prior HO and earlier Definition2 main drivers.So resolution at all frequencies is not just perceptibly higher than in prior Definitions -- the difference is a revelation. Further, the top-to-bottom integration of driver elements into a single behavior is advanced, given the lighter, stiffer FRD cone coupled to a torqier motor, the single strong 12" downfiring sub with the more competent Hypex sub amp, and the extended beautifully open Radian supertweeter, sonic behaviors are nearly full-range electrostatic-like in sonic holism, but with big dynamic range, high power handling and solid bottom end foundation.
I have heard the Merlin TSM but not in the same room and on the same gear as a Zu speaker. It's a good speaker. Personally, I can't go back to a crossover-based speaker after owning Zu. Leaving passive crossovers behind is a one-way street for me. The disjoin is distracting to me, but not to everyone else. That's up to you to decide. But I certainly wouldn't consider the newest Zu to be resolution limited on top relative to Merlin. I also don't think the older tweeter and HO driver in Soul Superfly are limited either in that respect either, in a smaller, less expensive speaker. But I think you can expect Zu to put the Radian and the new nano drivers into more upper-range speakers over time, giving buyers access to that sound in a wider range of prices than the $12,995 Definition 4 alone. But in all Zu speakers that get the nano driver, this goes a long way to overcoming the resolution difference some people have commented on in the past, relative to certain conventional speakers.