Your dealer is speaking nonsense, or at best only part of the story.
Most modern speakers have a too lean a tonal balance, because of the pursuit of small size for marketing / domestic acceptability purposes. Most modern speakers are less neutral than old school full size designs. Most modern speakers sound less open than old school high efficiency (greater than 96db/2.83v @ 1 metre) designs.
If you're just going to restrict your choice of speaker to small modern ones, then soft dome tweeters makes some sense.
If you don't have such restrictions, there are plenty of speakers that have used metal tweeters, or cone tweeters or compression driven tweeters that are relatively neutral and sound great with rock music.
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I think that typical rock recordings, eg Rush, Led Zep, Bachmann Turner Overdrive, Nirvana sound fine on systems with decent bass extension and clarity. On lean systems they can sound like bad recordings.
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I'm with you on Dynaudio. I've not liked any of their speakers that I've heard.
I think that the trick for you, peterpan is to find a speaker system that has a decent amount of bass extension, tightness and clarity, as well as having decent midrange and treble clarity. And preferably something that sounds dynamic and not shut-in.