Stand alone tweeters

You may not always be able to hear them directly that well but you can hear the effect. Commentators say they can't hear the Townshend supertweeters at all when they put their ear up to them, but when they are turned off, the difference in the total sound field is quite marked. Apparently they work even if you don't have a lot of direct hearing above 10Khz, it is not a "direct hearing" kind of thing, more a matter of how the high frequencies effect the sound field and psychoacoustics.
Have a friend switch them in and out while you listen. If you really can't hear the difference, then its a long run for a short slide, but you should hear the difference.

They work. I use Aurum Cantus ribbons with custom foils, but I use an active crossover and a separate amp. I think the issues are speed, dispersion and harmonics. Even if your speakers have response past 20Khz, are they really really fast above 10KhZ, and do they project that spectrum with extremely high energy and dispersion? Most diaphragms just are not fast enough, and you can hear the speed limitations.
The supertweeters add illumination and detail to the imaging.
You may not be aware of it until your turn them off and notice some of the light go out of the stage. Of course, the source matters, vinyl benefits most, but Redbook and digital can also benefit.