I've mentioned why the "rolloff" exists several times. It's not a typical FR rolloff like what exists in band-limited analog products. It's due to the beat/modulation, caused in most part by a sample rate that is too low. Since the signal modulates, peaks are sampled might be close to flat or maybe even lower than what the FR curve would suggest. That curve only suggest the "average" output at HF. But subjectively, the HF does not seem "rolled off", relative to a digital filtered signal that's flat to 20 kHz....... Since the time response is improved, the HF even seems greater in some cases.
When it comes to digital audio playback, the measured "HF rolloff" only tells half the story.
