soft dome versus hard dome tweeters

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The best sounding tweeter I have ever owned was in a speaker made by Harbeth, it used a Seas aluminum dome with ferrofluid cooling. Cymbals had a magical shimmer to them, midrange had additional detail and the integration between woofer and tweeter was seamless and coherent.

Aluminum domes often take more work and crossover parts to make them sound so exceptional- namely the need for low pass filters on the very top end to squelch any rising breakup distortion.

Get them right and they are hard to beat. Get them wrong and they are impossible to listen to.

I bought two pair of ADC 303AX speakers a while ago and upgraded both of them with soft dome tweeters and rebuilt crossovers and it totall transformed those speakers into something that was quite listenable and even desirable. One pair served in my office for a while as my standard speaker and I loved the acoustic suspension low end plus the soft dome tweeter (ala parts express). It removed the trashy top end the stock tweeter had and replaced it with one I could live with long term. Yeah it worked well. The ADS 303 is a good speaker stock but replacing tweeter and crossover makes them really good. Not ridiculous cost and very satisfying afterwards.

I also have had more recent experience with SVS bookshelf speakers with 'diamond coated' tweeters in their Pinnacle bookshelf speakers. Whether you have heard these or not, they are worth a listen as a decent little budget speaker. I have these combined with a pair of HSu subs in my bedroom and I love em. Used primarily for TV and at moderate levels they shine like diamonds :). I have never been a fan of small two way bookshelfs but given that I am living a new and different reality than what I had before my home burned, these meet a need and do so nicely. I do not find the diamond coated tweeters at all fatiguing but yet the detail and air is there. No complaints. I have an older pair of JBL Arc 90 3 ways that have aluminum dome tweeters...these were once part of the bedroom system and had a good clean crisp top end without being overdone. Overall, that particular speaker seems bass-shy to my ears and really needs a sub. Its not so much the tweeter is too brite, its that the low end is insufficient. Almost any bookshelf speaker I have ever heard short of the late great AR3a was deficient in the bass department. I would even say that about my AD303ax speakers, but they are fairly well balanced top to bottom. IF the Lord allows it I will keep the ADC speakers the rest of my days. They are benign with no glaring bad habits other than a lack of really deep bass. The JBL Arc 90s I could let go...not wonderful, not awful. The SVS are keepers particularly for secondary use. The SVS fit what my friend often says about something "Good enough for who its for." That is not damning it with faint praise. That's a compliment that says its a good match. 

A lot of what passes for common wisdom for hard vs. soft domes is like 20-40 years old. 

It's important to consider placement as well.  Speakers with hard domes that demonstrate on-axis ringing sound very good off, and are often recommended for minimum toe-in. 

I think rather than debate this endlessly audiophiles should consider a variety of examples, like for Be listen to Magico and Focal.  To my ears the tweeters sound very different and demonstrate how much the motor as well as the overall integration with the other components matter. 

Lastly, I've said this before, audiophiles are obsessed with tweeters.  I too have spent TOO much money on tweeters, when the reality is that so often we'd barely noticed if they just stopped working.  Point is, they just don't generate that much of the music we hear.  Better debates IMHO are what kind of mids you like, and how off-axis power response matters. 

My experience is that it has less to do with the tweeter material and more to do with the crossover.  Many say Be tweeters are harsh but my Rockports and anything but harsh.