I understand these NOS filters are incorrect or poorly engineered. However:
... In recorded music there should be no signal over 20kHz so any aliasing should only exist above 24kHz.
I have several recordings having energy above 20 kHz. I've also recorded some myself. It's not all that unusual, especially for close-miced recordings of castanets and other small percussion like instruments. Sometimes trumpet and a few other instruments. Of course that doesn't mean it's audible, but the energy is there, at least in some recordings depending on the mics used and positioning.
The weird part is that such 'speed' in music resides in the 2-5kHz region so any perceived 'improvements' in attack cannot be caused by the filter.
In fact treble above 5kHz may be a bit too low occasionally.
IME, ABX testing how filters affect high frequencies, from 12 kHz on up the difference is not tonality (as it couldn't be, since the harmonics are supersonic) but rather the timing or transients sound smeared when listening to castanets, jangling keys, etc. So for me personally, "speed" or timing consists in those frequencies near the upper thresholds of perception. Not 2-5 kHz, but above 12 kHz.
The so called 'NOS emulating' or slow filters do not adhere to the sampling theorem. Using it (and risking HF noise) is a choice of the user. ...
Here's where I have a question. Intuitively, without a filter, you're reconstructing little stair-steps defined by the samples. Each of those has infinitely sharp corners (discontinuous 1st derivative) which cannot occur in nature. Each of those corners forces a spray of high frequencies into the spectrum to form that sharp transition. The sample rate determines the highest frequency the samples can encode, and thus the lowest frequency of noise that must be filtered. If you oversample 44.1 4x to 176.4, then Nyquist is 88.2 kHz, so any noise must be > 88.2 kHz. For that noise to alias in to the passband, it would have to be at least (88200 - 20000) + 20000 = 156,400 Hz.
If so, then oversampling at high rates would obviate the need for a filter, since the HF noise would be at frequencies above the bandwidth of the amplifier. What am I missing here?