I heard Randy's late development speakers and the production versions now and then since. He has refined sonic judgment for what sounds good and his design principles are solid -- simple as possible without going all the way to an FRD, 2-way, 1st order crossovers, premium parts and materials, reasonable prices, EBS. Read his design page on the Sonist web site. Both the Concertos are well thought-out speakers. Plus, he designs for the right kind of relentlessly musical amps -- SET, SEP and modest-power tube push-pull.
If you value frequency linearity over dynamic life, you're likely to enjoy Sonist speakers. If sound is incomplete without speed, explosiveness and punch to go with tonal accuracy, you might prefer something else. They sound less dynamic than their efficiency suggests. Of SET amps, an 845 tube has the best chance of waking them up. A pair of Quad II or Classic II tube monoblocks would sound beautiful, in the push-pull realm.
Aside from preferring not to have a crossover smack in the middle of the listening band, the "not-quite" aspect of Sonist for me is the discontinuity dynamically and tonally between the ribbon tweeter and the mid/bass driver. Randy does a pretty good job reconciling the behavior differences of the two driver types, but can't close it. No one has. This is my reservation about the Zu Essence as well, and it only has the ribbon on a high-pass filter above 12kHz. Matching it to a toneful paper cone in a two-way with a conventional crossover point exacerbates the problem I already find distracting in the Zu Essence. But plenty of people like this combination and if you do, Randy's done a great job designing and making tube-friendly two-ways that make music in real circumstances and offer extended range sound.