Many vintage Infinity and Acoustic Research speakers (among others) are common examples of speakers with low impedance ratings, sometimes very low.
Since their woofers used the acoustic-suspension system (i.e. sealed enclosure) they tended to be power sponges vs. speakers using a bass reflex system. Advents were also sealed but they didn't use quite as much power since they weren't designed to go as low as the the higher-performance Infinitys/AR models.......and the lower you go, the more power you need to get that woofer cone moving.
To get that subwoofer-like bass, the engineer allowed the the larger models, especially those from Infinity (
http://www.infinity-classics.de/infinity/models/Reference-series-1+b-2-10+A+B-1981-86/index-Reference.htm) and AR's ultimate model the AR9, to dip into the area below 2 ohms which meant you needed to use one heck of a beefy amplifier to hear their full potential.
One mid-fi company still believes in acoustic-suspension: NHT. Not surprising since their founder used to work for.....Acoustic Research! Most of their offerings are sealed, though their very nice Classic 4 floorstander (
http://nhthifi.com/2006/s-s-classicfour.html) has its 10" woofer operating in a ported enclosure. It makes it all the way down to 27Hz - wow. Using a sealed system to reach that same number would most probably drive its rather low 86dB efficiency rating even lower.*
16 ohm speakers were popular when tube gear ruled, since a tube amp is a voltage-sensitive device and so develops more power when presented with a high resistance speaker (in contrast SS amps are current-sensitive).
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* off-topic rambling (hope that's O.K.): I always wondered if this fussy - and expensive - characteristic is what helped contribute to the near demise of the acoustic-suspension system. While to my ears, sealed speakers almost always have a smoother and richer quality than similar-sized bass reflex speakers, having to blow $$$ on a 100+lb SS power amp to properly power the larger models wasn't exactly a positive selling point. And even though Advents, EPIs and other less pricey loudspeakers didn't require such large amps to sound good, starting in the 80s there seemed to be an emphasis on how loud a system could get, rather than its sound *quality* :(, which I think also helped add to the unpopularity of sealed speakers.
To use an extreme contrast to that Classic 4, Cerwin-Vega's CLS10 (
http://cerwin-vega.com/CLS10.php), also using a ported 10" driver, has an efficiency rating of 90dB and makes it down to only 38Hz....but they also require less than half as much power to do this (every 3dB increase results in half the power needed for the same volume level). Yes I like CVs :shh: for what they are and think they have their place alongside the more staid speakers of the audio world, and I'm sooooo tired of seeing anemic little cube + subwoofer systems being touted as the best way to hear LZ, The Clash, Tool etc etc. and this toned-down CLS series has much less of the "party boy" sizzle other CVs are known for. And AFAIK, Cerwin is the only company left building traditional floorstanders like these for us old R&R geezers. :D
* I just noticed CV finally altered these speakers' styling slightly, and for the better: the previous version had a rather unlikable "grimace" because the bottom lip of the tweeter horn tilted downwards giving the impression of a scowling robot - now it's perfectly rectangular (this isn't a true compression horn system, since it uses a conventional soft-dome tweeter supplied by Vifa, to produce a more mellow personality vs. the typical metal diaphragm)