Amplifier stability with very low impedance, high efficiency mid/tweeter section???

 This is simply for a speaker I'm thinking of making myself, not for an existing product.  Once I model it I'll have a better idea of the impedance curve. 

@poseidon1500 To prevent the array from being low impedance you can put the midrange drivers in series rather than parallel. The efficiency does not change although the sensitivity does. A lot depends on how you design the crossover and the selection of the midrange drivers but that becomes a simple engineering issue. It would allow the speaker to sound smoother (all other things somehow being equal...) regardless of the amp used.

Could you explain the part about feedback falling off in the audio band as frequency increases? I have no idea what that means.  Something to do with negative feedback diminishing and thus THD going up?

You stated it correctly here. When the distortion increases above a certain frequency, you can count on harshness as a result. Combine that with an amp that easily doubles power as the load impedance is halved, driving a speaker where the high frequency array might be only 2 Ohms... that sounds like its going to be bright and harsh to me.

The technical explanation of why the feedback reduces at high frequency is quite technical! Its related to the gain of the amp circuit (with no feedback) and also its overall bandwidth (again with no feedback). That is why its called Gain Bandwidth Product. At low frequencies almost any amp has the GBP to support its feedback and  that is one reason solid state amps tend to have good bass. But at some higher frequency the GBP falls off, preventing the feedback from being the same as it is in the bass So the distortion increases. Amp manufacturers are a bit reluctant to publish this information since it tells you a bit about how the amp might sound.

All I'm saying here is that you have to be careful about the amp you select to work with a speaker like this! Put another way if the speaker were to have the usual 8 Ohms in the MTM array it would likely sound smoother for no other reason than the amp driving it will have less distortion.

But take an ESL, which may be 4 Ohms in the low range and 1/3 of an Ohm at the top. The combination of rising output impedance and dropping speaker impedance cause the amp to no longer give a flat frequency response. You lose treble.

@erik_squires You might want to leave ESLs out of this since they are not 'voltage driven' loudspeakers, unlike most box speakers. Their efficiency is not mapped by their impedance curve. So you need the same amount of power at 10KHz as you do at 100Hz to obtain the same sound pressure- and these two frequencies will be very different impedances! If you put an amp that acts as a voltage source on a speaker like that it will tend to be bright and lack bass impact as a result. There are exceptions- for example the Sound Lab ESLs have controls and switches on the back to allow you to address this problem.

@poseidon1500 There is not an amplifier made that will be harmed by the low impedance of your midrange/tweeter array. But there is a different issue which you might want to take into account!

All amplifiers make greater distortion into lower impedances. In addition, the ear uses higher ordered harmonics to sense sound pressure and so is keenly sensitive to their presence either natural or generated by amplifier distortion. It interprets them as 'bright and harsh'.

To complicate matters the ear has its greatest sensitivity at higher frequencies: between about 4-7KHz. This sensitivity is often called the 'Fletcher-Munson curve'. Something to keep in mind is that if you have an amp that can double power as impedance is halved (IOW, acts as a voltage source), harmonics of the bass notes will be audible coming from the high frequency array. This will contribute to brightness. Of course this will always be the case, the problem is that with many amplifiers as frequency is increased, there is a certain point where the feedback falls off inside the audio band. This causes the amp to make more distortion and this is very likely at or near the frequencies your MTM array is operating. If you are planning a conventional solid state amp this really sounds to me like a recipe for brightness and harshness.

If it were me I would insist on auditioning the speaker and amplifier combo in your home!