soft dome versus hard dome tweeters

Yes, implementation matters a great deal. It's impossible to "perfectly" isolate all other variables to do a "fair" comparison to these 2 tweeter types. I do have one experience that sticks out - I've heard all the Acora spekears at my local dealer (who sadly is now retiring). The most musical, enjoyable experience I've had is with the smaller SRC-1 tower. That's their ONLY model with a soft dome tweeter - all the others use beryllium!

I specifically listened to SRC-2 (extra woofer, and beryllium tweet) and SRC-1 on the same day. The SRC-2 had the advantage of higher-end VAC gear (much more $$$$), while the SRC-1 was on older (but still very, very high end) VAC. SRC-1 had the advantage of a smaller room, which frankly I probably prefer to the larger room in this shop where the SRC-2 were situated (so that's a big factor). 

Anyways, I really enjoyed the SRC-1 with its non-exotic textile tweet. I'm a die-hard Tannoy fan, but this was one of those moments that had me thinking I could be happy with a non-Tannoy. I still "like" the sound of beryllium (also on some Focals, like Sopra 3), but certainly there's a certain "hardness" to it that I suspect would not fare well for me in the long-run. This character even pervades into the Focal Utopia and Stellia headphones. I've owned the Utopia headphones twice, and sold them. Certainly, the best headphones in my experience all use "soft" diaphragm materials. The issue of ear fatigue is much more severe with headphones.

And that brings me back to my Tannoys, which use very non-exotic (aluminum or aluminum & magnesium alloy) "hard" tweeters. However, the implementation is quite different than most - with either the tulip wave guide or pepperpot horn / compression driver. Careful gear matching is required so the tweeters don't "bite" my ears - more so with the pepperpots. But when properly accommodated, they don't have the hardness of beryllium. And the end result is well worth it, to me :) 

In short, I'm skeptical of exotic "hard" tweeter materials, as far as their contribution to musicality and value to long-term enjoyment.

I want to repeat myself a little though. Tweeters don’t matter. They are the sprinkles on top of the doughnut.

Midranges matter.

I’m saying this a little funny, tongue in cheek, but honestly we pay far too much attention to a device that may not even be working in some speakers, and we overspend based on the tweeter.

Over here in Tannoy land, tweeters extend far into the midrange - crossovers are typically ~ 2kHz (tuplip DC) or 1.1 kHz (pepperpot DC, with compression driver tweet). Fyne, in their continuation of the DC design, has pushed crossovers even lower on some models, as low as 900 Hz (!!).

Now the supertweeters (crossover varies from 14kHz - 22kHz) - those are definitely "just" the sprinkles on top, but they certainly have output into audible range, and you can hear their effect at your listening position. So I imagine a 4kHz crossover tweeter is crucially important to midrange, still.

About beryllium domes - perhaps their durability / brittleness is a limiting factor for alternate implementations? We’ve only seen them used as direct radiating domes. Perhaps they cannot withstand the forces in a horn / compression chamber? The Focal Utopia headphones are interesting becasue there have *definitely* been higher reports of driver failures with that model, versus other headphones. This is an application where the beryllium dome is asked to produce full-spectrum frequencies, 20Hz and up. In fact, with a vinyl source, you could send some very large-amplitude subsonics; I always wondered if that was a high risk factor.

@knotscott  Depends.  You should ask the fans, but one approach is to use a coaxial or coaxial-full range driver.  If you are willing to accept the beaming above a certain frequency though you can get get most of the benefits of being without a crossover at all.

Yep - there's simply no "perfect" solution or approach to desgning full-range speakers. All have drawbacks. Some constraints are more important than others, and this is driven by how our brain / ear system, works plus add some variation for our differences as individuals.

If you want a more "perfect" approach you can try single-driver headphones - this eliminates the crossover, eliminates the room (you have ear interactions but it's "simpler"), and renders true full-range all from a single driver. You can pick your driver material - hard or soft - plenty good examples of both exist (I prefer soft). On paper, the headphone experience should be amazing. BUT, it's simply not as satisfying as a good speaker setup - even the really exotic and pricey headphone gear - and I've tried. There are multi-driver headphones too, and some of them are very good, even dating back to 1970's AKG K340!

The Tannoy coaxial (DC) is based on the idea that SOME coherence between the 2 drivers is necessary, but not 100%. They're phase coherent, but only precisely at the crossover frequency. They are NOT time aligned. Their 15" woofers are probably beaming at the 1.1kHz crossover (even worse for the 15" tulip waveguide models at a higher crossover frequuency), but the horn tweeter helps cover for it, and at least the radiation pattern is symmetric (coaxial arrangement). IIRC there was a study that claims human hearing can appreciate the affect of EITHER phase coherence OR time alignment, but that having both netted no additional benefits. The brain is wild, man.