Anyone listen to Zu Audio's Definition Mk3?

"Initially this appeared to be at the expense of tonal weight, but on further analysis I just wasn't used to the massive speed of these drivers. The old drivers in the 2s sound earthbound on reflection."

I had the same concerns at first with the nanotech drivers, but as I grew more acclimated and juggled a few tubes to compensate for the new signature, I arrived at the same conclusion as you. The weight and body is there--the speed and nimbleness is just initially misleading.

Absolutely, Gopher. So, I've covered the all important FRDs. Now onto the bass. Any reservations as to 4 x 10" woofers being replaced by a single 12" dedicated sub were blown away by the rapid realisation that the full bass extension of the 2s had been retained with a greater speed, agility and tonal differentiation in the 4s. Finally I'm able to integrate the bass much better into my room, and vitally, it integrates itself much better with the FRDs. This really makes for a seamless mid-low spectrum, and much more spatial and temporal microdynamic presentation, especially revealing of bass ambient clues (eg the edge of organ notes as they reverb in the recording space) and bass impact (eg differentiation of individual kick drum/piano hammer strikes). The boogie/party factor is all present too, with fantastic PRAT. Rest assured, the bass of the 4s is of a whole magnitude better than the 2s, and you will not find it wanting. I'm sure the more substantial weight/bracing/plinth of cabinet is making a major contribution here.

Sorry A'goners for my delay in my last posts re the Def4s, but "real life" intruded over the last week. I've found that as I've gotten older, and family/business issues become more serious and intrusive, I more and more really crave the serenity that a great audio system gives me, and my stress can melt away while listening to my regular diet of 3 albums a day.
Anyhow, covered the FRDs mids and bass, last thoughts are on treble. Here I'm a little less decisive in raving about the 4s. This is only since previous comments stated it to be qualitatively a leap beyond the 2s whereas in my room it is more of an evolution.
What I always loved about the 2s was the absence of the sort of spotlit treble evident in so many high end spkrs, and I do feel this characteristic isn't changed dramatically by the 4s. Now I know that this is a flavour imposed by the FRDs, and since the mids are SO much more transparent and delicate in the 4s this is likely what I'm experiencing with the treble. Again, high frequencies aren't overt until dominant in the program material, and then the music soars into the ether.
So my final conclusions. This is an AMAZING speaker, and a leap beyond what the 2s were able to offer. Most importantly, the new FRDs really introduce an electrostatic like transparency and delicacy to the addictive tone dense sound that we all love about the 2s, with no down side.
Bass is really impactful, more tuneful and much more integrated into the whole. Treble is less noticeable as a step up, but this may be the point: the FRDs covering it are so brilliant on mids on up that this character into the highs is all part of a subtle blend.
I was SO fearful upgrading sound unheard, but all has been allayed. The 4s are an absolute giant killer, imho really treading onto the territory of uber spkrs over 4x the price, but maintaining and enhancing the addictive tone dense Zu sound that no other spkrs seem to approach.

I would be interested in how well the Def. IV comes alive on lower spl levels and how well its resolution is overall as compared to more conventional speakers. Can you shed some light on this please?

I believe it's resolution at low levels is exemplary, a lot to do with it's famous tone dense presentation, high efficiency and lack of crossover.
Density of tone means that the sound is fully fleshed out from low, late night volume levels, whereas with a lot of more expensive speakers (the Kharma's esp IMHO), the sound only "gels" at reasonably high SPL's. So there is no real need to turn the wick up unless you want to, and music can really be enjoyed at intimate listening levels. Additionally high efficiency and an easy impedance curve means that low watt tube/SET's which excel in the more personal type of sound really find a great synergy with the Def4s (and all other Zu models I believe). Finally, no crossover means no sucking of energy out of the presentation, and with the FRD's covering a broad spectrum of frequencies, this adds to the holistic presentation at low to moderate SPL's.
All of this results in excellent resolution, the 4s being a real leap fwd compared to the 2s. Having heard Pro Acs, B&W's, Magicos and Wilsons etc. over the years, IMHO the Def4s give nothing away in terms of resolution.
But that resolution is portrayed in a different way to most other spkrs, cheap or uber expensive, on the market today.