I'll try to answer why I kept the Soul Supremes over the Druid 6's (not 5's, as stated in the question--I never owned the 5s, but heard them a couple times)...but my answer will be unashamedly subjective in nature. As scientific as I tried to make my personal comparisons, a bunch of things changed (including my rooms, and my exposure to field coil drivers) and I ultimately relied on emotion. And by the way, the Soul Supremes are for the moment in a 3rd system I own, and are going to be moved to a 5th system. So while I kept them, they have moved into a specialized spot for me personally....they continued to get trickled down to lesser-used systems. But I still love them...I think. But let me be clear--for the $, they remain a phenomenal speaker, especially at used prices.
Before I go there, however, while I understand Sean's comments that Zu's love high-power (I'm quoting the post, I don't remember specifically reading that, but I don't doubt he said it), that by itself probably explains where I feel like some of the design choices made at Zu might differ from goals of my own.
I'll just state my own bias...building a speaker that is by nature quite high (very high?) in efficiency (98-101db or whatever range they end up) and then having the feeling that they really need "high-power" seems like something was ultimately traded off that does not match my own personal design goals for a high-efficiency speaker. I am not suggesting that everyone should end with flea watt SET amps, but I certainly do not personally feel that moving to "high-power" amps is an end-game answer for any high-efficiency, full range driver'd speaker. The trade offs in capabilities as you move into the 50 wpc or higher amp designs are, by nature, trade offs that are easy to hear in transparent high-efficiency speakers. Heck, even 18 wpc on 845 SET amps or a solid state SIT3 is easy to hear the trade-offs over something like 300B or PX4 and then again over 45 or 71a (when those amps have top designs and top parts). You lose some micro detail and nuance, minimally, even if there is a gain on slam and dynamics. Once you've heard a particular set of traits in those ranges, for a near-end-game emotional level, you'll find you need it all, not a trade-off one way or the other. I have owned (and still own) a very large number of amps in many topologies, solid state and tubed, and have spent countless hours with each in many combos. I'm not saying I know better than everyone--but I'm saying I have owned (and still own) about 10 different sets of high-efficiency speakers of different brands and eras, and about 30 amps to pair them all with. And yes, much of this is trial and error and subjective preference, so if this paragraph has offended anyone, please don't be. You may love "high power amps matched with Zus" and I won't challenge that for your personal situation. It's just not what I found in the end (and ultimately moved on from the Zu design, in spite of still respecting it).
The Druid 6 is a much more capable speaker than the Soul Supreme...don't misread my action of keeping one and selling the other as a comparison in that respect. Both are great speakers that have sweet spots for their usage and price ranges. The Druid 6 took a larger step away from the Soul Supreme vs. the Druid 5 did, which given the 5's parts similarity to the Supremes, the Druid 5 may sit closer to the Supreme than the 6. The 6 retrieved more detail, gave slightly better imaging, seemed to go a little lower, seemed to go a little higher, seemed to have slightly more black backgrounds, and more slam over the Supremes, all else "equal." None of that should surprise people. It was in my opinion more "high performance." Zu probably feels like they accomplished what they had hoped to do with the 6...make a new speaker that was largely based on their experience to date but with everything just a little refined, better, newer, tighter. It was "better" by the subjective (and probably objective) measuring posts. However, I can't prove my next comments, but I also think they were more difficult to nail in terms of positioning and gear to get the most out of them. They relied on a higher level of associated equipment (they could sound spectacularly average with the wrong gear and wrong room). If you didn't have the right room or the ability to pull them out into the room, they sunk a bit in terms of soundstage. I could say the same exact things about the Soul Supremes, but the range of their performance was tighter in those scenarios...it didn't get as crazy good so it also didn't have as far to drop when positioned incorrectly or paired with gear that was not as good as a match.
However, as good of a level as the 6s reached, I sold the 6s because they were not ever going to be end-game for me. That was a very personal and subjective decision and not a knock on Zu or anyone who finds them to be "end game." They are great speakers, and an improvement, but as you move into $10K and above speakers, the goal posts change for how amazing they need to be.
I kept the Soul Supremes because I love the look of them, they are capable of producing some very emotional and musical sound and seem to be a nice middle-ground from Zu between superb entry-level high-efficiency speakers that can be easy dropped into an environment and produce "fun" and what I suspect they intended to be more an "audiophile" speaker that tries to compete with the best. The Supremes retrieve enough detail to make your ears perk up and go "wow" enough of the time when paired with great amps and source, but they don't try to compete with $20K or higher speakers. For that reason I will probably continue to try to make a system around them to listen to on occasion. They could certainly be the cornerstone of a very top end system, and if you got them for $2K you stole them at that price...well done.
