By getting short notes up sooner, there are many aspects of Def4 I haven't yet commented on. It's true that the sonic spray into the room is wider. For me, it was already excellent with Def1.5 compared to most floorstanding speakers, though less broad than with the better point-source standmount monitors. But I had very good off-axis response and soundstaging in my room with Def1.5, which marginally improved with Def2. Def4 is a larger improvement in the distribution of full-frequency sound and soundstaging over Def2 than Def2 yielded over Def1.5. This is a function of improvements to all three driver segments. The supertweeter upgrade easily obvious in this respect, but the stiffer/lighter FRD cones spread better as well, and the monocone sub-bass makes its contribution to more even room loading as well. But while Def4 delivers a big sound when appropriate, it doesn't impose a spatially big sound when the recording or performance doesn't warrant it. The soundspace scales up and down to the music appropriately, yet the room is pretty evenly loaded acoustically even at low volumes.
WIth respect to power requirements: Sean Casey said at the time that upgrading Def2 with the 2010 pre-nano HO FRDs made the speaker perform more like it has 104db/w/m efficiency. He didn't change the rating on the speaker, but said the power transfer improvement with the HO driver would nudge the speaker's apparent efficiency upward. He was right about the "apparent efficiency" part and that probably was enough to make some small amps that were only marginally sufficient before, acceptable to some people in some rooms. Certainly, Def4 has an apparent efficiency that sounds dynamically higher than Def2 in its original form. There are multiple contributors. First, the nano main drivers bring the known strength of the pre-nano HO drivers. The supertweeter does a better job of keeping up with the main drivers as volume rises without losing clarity. And the sub is both cleaner and more articulate for being one driver instead of four, but also the new Hypex-based sub-bass amp has more headroom and it's "faster." So the net result is that with the jumpiness of the Def1.5 restored and furthered without the cabinet talk, I think some people can be happy with a smaller amp than with Def2.
That said, I still do not think that a 2w 45SET amp is enough for optimum general use, and won't allow a Def4 owner to hear the full capabilities of the speaker. But if you really like true flea-power SET, you're more likely to be able to live within its limits on Def4 than on Def2. I think the Definition architecture, however, is optimally driven by 12w-30w SET tube amps or the very simplest push-pull tube amps, of which the prime example is the Quad II monoblock pair, or some of the 300B push-pull designs like Audion offers. Seven to ten watts 300B SET amps are quite serviceable IF it is a design that dispenses with the bass bloat common in ordinary 300B SET designs, but that's really borderline with respect to experiencing the dynamic life Definitions can deliver if your room isn't small, IMO. It's not that seven watts can't drive enough average SPL. You hear the difference in the ease with which dynamic spikes are handled by the available headroom.
Nothing about Def4 changes my view that the ideal amplifier for it is 20-30w 845-based SET or similar big glass triodes. Some 211 amps in the 15 - 20w range are quite good too. At the recent L.A. Zu house party, I heard the Melody 211 stereo integrated amp driving my Def4s, and it was strong, agile, articulate and beautiful.
However, for people who like the broad creamy torque of a really large solid state amp, I think Def4 is even more accommodating of their sonic signature. For solid state, the McIntosh quad-differential autoformer-output power amps are particularly synergistic with Definitions. Electrocompaniet amps work well with Defs too. Despite the efficiency of Zu speakers, their high power handling gives you latitude other HE speakers can't offer. Put a pair of McIntosh MC1.2kw or MC600s on a pair of Definitions, and you will understand what the sense of "unlimited power" into a crossoverless speaker can offer in terms of sheer dynamic ease. Not everyone is ready for or oriented to tubes. Counter-intuitive to expectations and similar to the effect I'm hearing with CDs, I fully expect that the Radian supertweeter will present the grain and sometimes hash in solid state amps less obtrusively than the older Definition supertweeter. But this also means that low power solid state amps like Pass Class A, First Watt and 47 Labs should sound more beautiful too, than they have in the past.
A reviewer who is getting Def4s dropped by last week to hear them on my SET amps and on Quad II, which is what he will be using. If you've read anything here I've written previously about Zu speakers, you know that I advocate going heavy amplifier quality with any Zu speaker, and have said in the past that Superfly powered by a great $10,000 amp will sound better than Definitions powered by the best $1,000 amp. Given the quality of some tube amps coming out of China and on the used market, that may be less true with Def4. New production Quad II monoblocks list for $3300 or thereabouts. THAT is a hell of a lovely sounding combination where the amplification is reasonably moderate, the design is simple and retubing costs are light. Quad IIs sound considerably more muscular into Def4 than Def2, in part because of the new speaker's 8 ohms nominal load against 6 ohms for the older version.
Overall, I think Def4 allows a wider range of amplifiers to put it in its sweet spot, but I still think the centerpoint for optimum match is high quality 845 or 211 SET, led by (in order) Audion, Sophia and with Melody coming on strong right on their heels.
Phil
