Will you do your own crossover too? That seems to be a pretty exclusive piece to the Sierra. The tweeter is supposedly customized for Ascend. I have seen a similar tweeter on Totem's wall mount speaker.
As for good tweeters with typical monopole off axis response, very flat response, superb CSD and very low distortion, get a BG NEO 3 PDR. In addition to all of these traits you seem to want, it is an almost perfect resistive load (it's a planar), and can be used down to 2000Hz with appropriate crossover, making it extremely easy to integrate with a wide selection of mid-bass drivers.
The only notable thing about that tweeter, compared to what you can typically get DIY, is the waveguide. A waveguide (used on pro monitors for many years now) removes the edge diffraction disturbances that occur when the tweeter's sound pressure waves meet a sudden pressure change at the cabinet edges. The waveguide focuses the energy to be distributed more on axis, to remove the stray off axis energy. Of course, this further limited off axis response may not be desired, depending on the objective/end use of the speaker. But that's another topic. You can achieve the same diffraction reducing effect, with wider dispersion(a desirable thing in normal circumstances), by using very large radius on the cabinet edges (2"-3" for example). However, this can often lead to large increases in cabinet production cost, and as such, is a fairly rare feature in low and moderate cost production speakers. You can also make a waveguide if you have a lathe, and copy the profile of a known working unit. Or you can buy some plastic shallow horn waveguides, which are appropriate, if you shave the back off and mount the tweeter to the back at the appropriate depth. Zaph Audio DIY blog site has such an example thoroughly documented along with part numbers and sources. [
Note: I do not endrose the Zaph site, I only refer to it in the specific citation of the DIY waveguide]
The Seas tweeter used by Ascend may be a nice integrated package. But I don't believe it's available to DIYers yet, and I am not sure of the cost. Will it be cost effective for DIY, as compared to the alternate methods to achieve the same effect it offers?
Another issue here is the cabinet. Due to the inherent increased stiffness of the bamboo, it would be substantially less resonant than a MDF cabinet of equal construction dimensions/bracing. While still not likely to be non-resonant, it would likely be a large improvement. This needs to be considered in the DIY cabinets. Typical MDF box construction methods/bracing, IMO, is worthless, for a supposed high-fidelity loudspeaker. The reviewer also commented that the speaker used what looks like fiberglass board cut up and placed in bags in the speaker. Could this be rigid fiberglass board? Like OC705 or equivalent? If so, it would be rare that a speaker manufacture bothers to use extremely high quality acoustic damping material. Normally, a low density fiberglass, low quality thin foam, or low density Dacron/poly fill is used. Usually sub-optimal. This should also be considered in the DIY design.
The Sierra, according to the manufacturer, uses a very low distortion mid-bass with long linear excursion/incursion. The surround of the mid-bass certainly appears to be unusually capable, though this is only superficial evidence. But let's assume this speaker really does have the non-linear distortion vs. excursion performance of say for example, a premium Seas Excel driver.
Overall, it would
appear the Sierra is one hell of a well engineered speaker for it's retail price class.
-Chris