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RE: "OTOH you are probably stone deaf above 16 kHz" ...

Posted by Satie on February 15, 2017 at 16:35:28:
I had that sort of issue but didn't measure the voltages with the Bryston 4B NRB which I tried on the tweeters. It is rated to 4 ohms and has oodles of power, but only "stable" to 2 ohms. Though it did drive them, it ran hot and sounded thin. When I had the option I swapped with a MOSFET Fosgate amp that weighs the same but has 1/4 the power rating. Also rated to 4 ohms and manufacturer suggests cooling fans when running into a 2 ohm load. It was run with and without a 1 ohm resistor in series. Without a resistor the amp got hot and images became tiny. When it got hot to the touch I would turn it off and put the resistor back, which is how I ran it most of the time. But for that one time when I forgot and ended up burning the amp. I have another which I use on occasion, and always with the extra resistor, which I also use with my tube amp. The only amp that drove the tweeter with no problem at all was the Classe DR9. Which is rated to 2 ohms. Ran a bit warm just as it did on the 6 ohm mids. The PSE Studio IV didn't stress on the tweeters but ran hot.

I did hook up the Crown just to see what would happen. Which was nothing in particular, it sounded like the Bryston on top, which is as expected. But there was no heat, the fans never turned on and the load light was never lit. But then it is rated to 2 ohms and stable to 1 ohm.

So I see that a low impedance load can be a problem for an amp not designed for it regardless of power rating. Experimenting with different resistors on the Fosgate amps it was rather obvious that it did best when the load was over 4 ohms. Even though it was losing nearly half its output to the resistor it was never stressed and never ran hot playing single ended (40W) and definitely no problem in bridged differential mode (80W), which did not sound as good as it did in single ended mode.

BTW if you are wondering why I didn't use the resistor all the time it was because it was the OEM Maggie ceramic resistor and it sounded like $^#$T. It took me time to admit to the fact and order a nice mills wirewound piece. Once I had that at hand I did not run the tweeter amps without the resistor, except for the DR9 which didn't need them.

For the OP
Thinking further then it seems if the high freq impedance drop is a concern for your amp''s high freq performance then you should find the 20.7 a substantial improvement. But then, I don't know that it actually is an issue for it. I second Neo's suggestion to take the amp with you to the dealer's to try it out on the 20.7. While there ask him about any "sanctioned" conversion of the XO to biamp.

Though there are many other factors involved, I think that if your amp sounds brighter on the maggies than it sounds on the MLs then it is likely that its upper end FR is drooping into the low impedance load of the MLs.