Comb filtering happens when you have two drivers (speakers) playing the same signal and the signal from one of those drivers reaches your ear before the other one. Depending on the delay time, some frequencies are reinforced while others will cancel out, making the frequency response look like a comb with teeth (peaks and dips). This is a very audible (and proven to be audible) effect.

So while the super tweeter can give a substantial boost to the high end response, it can also degrade the midrange response. And our hearing is most sensitive to these midrange frequencies.

Not shown in this video is how much worse the response can become when listening off axis, especially vertically. As you move above or below the speaker's tweeter, the delay increases, causing more of that comb filtering effect.

Getting the super tweeter as physically close to the speaker's tweeter as possible and using a higher order filter (crossover) will help to mitigate these unwanted effects. But a better "fix" for a perceived lack of high frequency detail is to either crank up the treble knob (if your setup has one) or get yourself fitted with hearing aids.
Believe it or not, hearing aids will make a massive difference if your hearing has degraded due to age or damage. And it's a difference that you can't match by boosting the high frequency response of your speakers or by adding a super tweeter.

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