Great 16 ohm Full Range loudspeakers

I am considering getting a second set of monoblocks--Atma-sphere MA-1 140 watt blocks, and am interested in learning about the available 16 ohm loudspeakers out there. I would want something that is full range (currently using JM Lab Alto Utopias).

why 16 ohm?
Check out Coincident, He was making 15 Ohm speakers. The Alto Utopias are a great speaker with good sensitivity and I think a min of about 3.6 ohms what's wrong?
Why don't you ask Ralph? I know he has been into C.A.R. speakers based on the JBL Hartsfields.
Most OTL amps are happiest with a fairly high, smooth impedance curve. That being said, in my opinion the MA-1 will work very well with quite a few 8-ohm (and lower) speakers as long as the impedance curve is fairly smooth.

I don't know what the impedance curve of the Alto Utopias looks like, but if it's similar to that of the Nova Utopia then it may well have a 6-to-1 spread above the bass region. We're used to thinking of impedance dips as being the biggest problem, but a peak can be a problem too. Let me explain:

Most speakers are designed on constant-voltage (solid state) amplifiers. If you have an 8-ohm speaker that has a 16-ohm peak in the midrange, chances are it was "voiced" with the expectation that the amplifier will be delivering only 1/2 as much wattage into that peak as into the 8-ohm portion of the spectrum. But the MA-1 is in effect a constant-power amplifier; in other words, it will deliver rouighly the same power into that 16-ohm impedance peak as into the 8-ohm portions of the curve. So as a result, the speaker is about 3 dB louder in that region with the OTL amp than it would have been with a solid state amp. If the peak happens to be around 3 kHz, as with the Nova Utopia, then that 3 dB peak can make the speaker sound too forward and possibly even a bit edgy. Most people will mistakenly blame the amplifier, but actually it's just a poor amplifier-speaker matchup.

Going back to that possible 6-to-1 impedance spread, we could see a 7 dB peak variation in the frequency response of the speaker when going from a solid state amp to an OTL amp.

Now if the impedance curve is very smooth, the tonal balance will be pretty much the same with a wide range of amplifier types. In practice we usually still have impedance peaks in the bass region, which can actually work to our benefit in some cases.

I have some experience driving both 8 and 16 ohm smooth-impedance speakers with OTL amps as well as other types of amps, and in my opinion many amps sound better with a 16-ohm load as long we stay below the onset of clipping. The disadvantage of a 16-ohm load is that a solid state amp will voltage-limit and clip at about half its rated 8-ohm output, and a transformer-coupled tube amp that doesn't have 16-ohm taps will also deliver reduced maximum power into a 16-ohm load (but it may well sound a bit better while doing so).

DIY a speaker with PHY km30 SAG John K- K.C.S. Speakers
Tonian Labs makes 16-ohm speakers that use both PHY and EAR drivers.
Atec Valencia 846A has a 16 ohm version. The humble Duke also did not mention his Audio Kinesis speakers. Audio Note make a High Efficiency version of the AN-E, designated HE. However, I believe it's 98DB, but not 16 Ohm. I think the AN-E HE's are actually an 8 ohm load. Well, I guess the Audio Notes wouldn't necessarily be full range.
Does seem Dukes speaker impedance would be OTL friendly.
Second the Altec Valencia 846A, the 16 ohm version, although the 846B (8 ohm version) is also very good. If money is not a consideration, then my first recommendation is the Shindo Latour's. Best overall speaker I have ever heard.
The Audiokinesis Dream Makers are 16 ohm and the Jazz Module 12 ohm. Both are OTL friendly. The MA-1 amps could easily power either speaker.
The MA-1 is very comfortable with 8 ohm speakers and a good number of 4 ohm speakers.

In fact its smaller brother works fine with 8 ohm speakers too.

Impedance peaks can be dealt with by Zobel networks if the designer of the speaker was expecting the peak to reduce the output from the amplifier.
Zu Soul superply, 16ohm, 101db efficiency. I haven't heard them but I have other 12ohm Zu speakers which I like.
Duke, I will be at RMAF in October. what will you be driving your speakers with?

Vandersteen 5A loudspeaker design has an even Imp load as well as (Hi passed Bi Amp)which takes the burden off the tube amp on the lowest bass with its own self contained internal bass amp and features adjustable Room compensation.
This gives us the tightest full range bass imaginable while allowing tube amps to recover faster last longer and sing.
Will work extremely well with these amps and there size is about the same or smaller as when you have.
I will be showing with Atma-sphere and Neko Audio in room 1102, and amplification will be the S-30.
A local SD company makes Audience speakers.
The writeup in the local rag is pretty neat with some photos. Crossoverless, full range line array standing about 6' tall for 'only' 54,000$ and the 2 speaker bookshelf version for a mere 5 grand.

No matter what the impedance, they'll probably be a very benign amp load.

phy km30 sag. buy the drivers, build the ob cabinets, and just enjoy the music...
I used to live with Ocellia Caliope .3 silver but I could not stand it's slow, muddy & thick character, so after 4 years of changing every posible link in chain, I give up & trade it.
I now use a Carfrae Little Big Horn with Lowther PM6A Ticonal.
My best ever speaker is the Shindo Latour.
I'm not saying these to confuse you but you must to realise :
The speaker choise is a strictly personal hit and everyone of us adores a speaker that someone else can strongly dislike.

Your amplifier of choise is a great upgrade for every speaker, providing that you are going to purchase the ZERO transformer from Paul Speltz.
After doing this, you can choose every speaker you like for pairing your WORLD CLASS POWR AMP.!!!

I forgor to mention that the PHY KM 30 sag (that my Ocellia Calliope uses) has a max power limit of 25W so, I feel it is better to look elsewhere (even if the open character of your MA1 can manage to cure some weight problems of the Ocellia).

Also I must confirm that different implementations with this driver can give extremely different results. So, it is not any correlation between Calliope & Open Baffle or anything that K.C.S cabinets can provide.