Most high-quality loudspeakers are 4-Ohms

I agree with Swampwalker - most really high efficiency speakers are 8 ohms or even 16 ohms. There are a great many of us who prefer these mostly vintage speaker types (that were designed for tubes, not solid state) to most if not all of the modern designs.

Preposterous generalization.
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Learsfool, no that I totaly disagree with you, but I don't think that Swampwalker actualy posted that.

A designer would need to invest more in drivers for higher ohm designs. Not alot of cheap 16ohm transducers about even in pro audio they cost more. But many cheap 4 ohm and 8 ohm transducers. The designs that are not Hi-eff are the far more compramised designs. They are designed to fit so many per pallet as much as sound quality. The profit margines are consider as much as performance. They have to apeal to many and must have hi SAF plus be easy to stock ship. Hi-eff designs are built mostly by small companies who realy care about end result. Since they move very few units they need to realy care about what they do since the moneys realy not there you get a labor of love so to speak. Or you can buy a 4-8 ohm design built in China with a vaildated audiophile aproved name. You get cheap MDF cabs and weak motor drivers. These will be placed in small bookshelf or slim towers with multiple woofers. Boring....

A lot of high quality speakers rated 6 or 8 ohms overall seem to have impedance curves that in fact go to 4 ohms or lower at lower frequencies in particular. I haven't counted so I don't know if most do, but a significant % certainly do.

Of course there are other high-quality designs that are well above 4 ohms as well.

I don't think impedance alone is a reliable indicator of "high quality", assuming you take proper matching of the amp into consideration, which I suspect many do not in practice.

My guess is that most speakers being built today with impedances of greater than 8 ohms (a significant minority overall I suspect) are high-quality whereas speakers designated 8 ohms or less are more hit or miss in general.