How can I answer this without sounding flippant? ... but if you can no longer hear a difference in the top band of the EQ, it might be time to consult a hearing doctor... (seriously).

Or it might be time to learn something useful about musical Frequencies (and lower your expectation of just how much you should hear) 🙂 ... the “High” Frequency Band can be set to frequencies as high as 16kHz which is almost 2 octaves above the highest key you can play on the piano. I’m not sure how much of a change you are looking for, nor am I sure you’re listening in the right area.

The lowest note on a standard 88-key piano is A27.5Hz
The highest note is a C4186.01Hz

If your High Frequency Slider’s default Frequency is centered around the 8kHz Frequency area, you should hear very little change on the piano notes directly. Play some music (put a .Wav File on a usb stick, for example) with some cymbals (Hihats) if you are playing acoustic piano notes listen for high harmonics they are the only things being affected by the High Frequency slider. And you should still be able to hear them ...so (seriously) check on your hearing... (try reducing that frequency see if you hear a lack of high frequency.) but the fact that you are using the piano sound shows me you don’t know the range of the instrument or what to expect from the High Frequency slider... try it on some full frequency music then Let us know... I’ve never heard of the EQ not working - I’d be very surprised if that were the case.

Also in play is your speakers and their ability to reproduce frequencies affected by the High slider. Example, a typical keyboard amp with a piezo and a woofer, everything above 4kHz is in the piezo. What if it is blown? You will not hear a change at 8kHz at all. So before blaming your ears (or the CP4 Stage’s high quality EQ), It could be your speakers... check with a good pair of studio monitors and headphones before concluding its the EQ or your ears.

Just FYI the “High Mid” freq band typically is centered at about 3.2kHz so yes!! the Highest Octave from C2093Hz (C6) to C4186Hz (C7) is right in its wheelhouse.... while 8kHz (High slider) is well above that...almost an octave above.

Just FYI... There are a lot of silly videos on YouTube... just fyi

Well you are correct in that I have much less ability to hear high frequencies than when I was younger. I hadn’t considered, but should have, that the equalizer was controlling a range above the upper octave on the piano. A little more thought on my part would have made that obvious. Thanks for the education.