Thg said: "I think that you should have no response below 15K Hz in a supertweeter."
While that is a nice sentiment, it is highly unlikely.
The Murata is rather vague about its crossover mechanism but if its range is given as 15KHz and up, it almost certainly has output below that (just like the other two supertweeters mentioned.) If it is a simple capacitor, the crossover slope would be 6 dB. Mechanical characteristics will likely add to that slope at some lower frequency but we don't know where that is.
If it is a 6 dB slope, that means output could only be down 6 dB at 7500 Hz.
Even if there is some super-high frequency magic taking place, it is pretty easy to see there is also a fair amount of energy being added in frequency bands that are already being serviced by the main speaker.
Back when I worked in a stereo store in the 1970s almost all equipment had tone controls. It was nearly universal for a customer (regardless of system, age, type of music or any other factor) to turn the treble up a notch or two. They just liked the way it affected the sound.
I hardly find it surprising that an add-on tweeter that clearly has output in the 10KHz band (even if the specs indicate it starts higher) is going to be audible.
There is an old saying that when you hear hoofbeats you should usually think of horses before you start looking for zebras. Maybe there is a zebra out there at 40 Khz but I sure see a lot of horses in the clearly audible 10KHz to 15Khz range in all these products.