I recently shepherded a friend here in Los Angeles into Definition 3s, his first Zu speaker. He is driving them with a Cary SLI-80 KT88 integrated amplifier running in pseudo-triode mode, with both CD and vinyl sources. He migrated to Zu from Triangle, after a series of unsatisfying trials with crossover-based speakers. I've been able to spend about 30 hours listening to Def 3s, considering the differences between them and my Def4s, while helping him dial in his system. A few concrete observations:
1/ The nano-FRD, as sonic anchor to both Def3 & 4, gives both speakers the same essential character. In either speaker, you get the speed, agility, openness, elevated articulation and tone density that the nano driver delivers over the prior lively Zu FRD iterations. The tonal darkness of Def2 prior to the 2010 HO drivers, over Def1.5, is gone. The Def1.5's burstier jump factor is back, without the MDF glare and other cabinet talk at high volumes.
2/ While Def3 is built in Def2 and Def1.5 cabinets that are reworked with more internal bracing and other resonance damping, the Def4 cabinet is still a different structure and it is "quieter" at high volumes. You'll not notice obvious cabinet talk from Def3 in isolation but hearing both in close succession, Def4 has the "blacker" background with nominally less cabinet presence when pushed.
3/ Def2 had only a level control for adjusting the sub-bass array's output, so Def3 having the full Def4 sub amplification and control panel puts the differences between the older rear-firing 4x10" line sub array and the newer 12" downfiring single driver, in sharper relief. Until Def4, Def1.5/Def2 deep bass sounded impressive, convincing and well matched to the fast dynamics of the FRD. In Def3, that's still true, with the added benefit of now having much more precise control over the deep bass output characteristics for mitigating acoustic problems in the room. But it's when you hear Def4's 12" downfiring sub that you appreciate by comparison its improved clarity of deep bass information for being consistent with the instrument making the sound, and the precision of single-driver unity over 4 drivers. The stun factor of punch from the 12" is also vividly greater. The Def4 further distances itself from Def3 by the superior evenness with which the downfiring sub loads the room with deep bass SPL. If Def3 and Def4 cost the same, you'd obviously want Def4. But they don't. Considering the cost saving of Def3, its sub bass array delivers better deep bass presence and defintion than anything else coming from a speaker of its footprint and cost, and it's better than Def2 or 1.5. So which one you want is simply a matter of what you can or want to pay.
4/ As with the differences in sub bass output, so it goes with the difference of supertweeters. Judged in isolation, Def3 using the prior Zu supertweeter, has a sparkling top end for harmonic completeness, that is well matched to the nano-FRD's whizzer output. The older supertweeter smoothly takes the hand-off for extension beyond 12kHz. But it is by comparison a ruffian against the exceedingly graceful Radian 850 compression tweeter. Your specific sonic priorities will affect whether you hear the downfiring 12" sub or the Radian 850 supertweeter as the greater benefit to Def4 over Def3, but to me it's the Radian that makes the Def4 in today's market a $35,000 speaker. That's not to say that Def3 doesn't deliver more comparative sonic worth than its price -- it does, like all Zu speakers. But the older Zu tweeter holds Def3 back from confidently disrupting the middle five-figures speaker market. The grace and effortless harmonic wash of the Radian 850 makes it probably the most beautiful yet truthful tweeter I can remember hearing.
If you own Def3 and want to keep your fiscal discipline, then only hear Def4 in someone else's system where you can write off the lovely top end and the exceptionally articulate bottom as resulting from other components in their system. If you are deciding whether to buy Def3 on value principles or extend yourself for Def4, these are the actionable differences. Decide what they are worth to you. If you have Def1.5 or Def2, either newer speaker will be an emotionally satisfying upgrade to your enjoyment of music playback. If you are migrating from another brand's crossover-intensive speaker, then any Zu will prove illuminating.
Phil
