Most high-quality loudspeakers are 4-Ohms

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.

JohnK, that some drivers that are high impedance design have little demand and therefore drives up their cost doesn't neccessarily make those designs worthy of their extra cost. On the other hand some of the more expensive drivers are indeed low impedance designs as well. That many drivers are designed to work with the most commonly available amplification is of no surprise, but the subject matter here is "high-quality" loudspeakers, and the those mass market drivers you are refering to aren't really what we're talking about here.

Nope. Mine are 16 ohms! I prefer easy-to-drive speaks with high, stable impedance and high sensitivity.

Mapman, I some level I think you've hit the heart of the matter with "I don't think impedance alone is a relaible indicator of "high quality", assuming you take proper matching of the amp into consideration...."
While we can't truly seperate amps from speakers, the OP question offers food for thought on the issue re: impedance and the quality of speakers. If we try for just a moment to stay focused on speakers alone, the OP's question isn't as silly as it might first appear.
I suspect that many speakers that are designed to be more effecient are in fact not so due to inherently superior drivers or other design parameters but, done so that they can better accomodate particular amplifiers and that course of action might just very well compromise the ability of the very speakers themsleves.
There are some here that think that less effecient speakers and big ss amps are silly. I would argure that that's no sillier than less efficent, small tube amps driving big speakers.
My experience tells me that the differences between speakers is greater than the differences between amps regardless of the amps technology.
YMMV, everyones tastes are different, and the marketplace has almost everything for almost everyone. Enjoy, regardless of the path you choose.