40kHz gives that razor‑sharp shimmer that lets the mix feel airy, but only if you keep it balanced. It’s not just a fantasy—when you bring the ultra‑high end in, it lifts the whole texture, but most headphones or speakers can’t actually play it, so you’ll hear it as a subtle lift, not a loud pop. Keep your cables low‑capacitance, straight, and make sure your DSP chain isn’t adding hiss or distortion. In the end, the real test is how it sounds on the stage speakers, not just the monitor. If you run a quick test with a sweep and listen, you’ll know if that 40kHz sparkle is doing its job or just getting lost in the mix.