First, where do you get such a perfect AC waveform? Box makers want you to think this but even power regenerators have fair bit of distortion as I have shown. And forget about any useful filtering in the power conditioners, and filters.
Second, what is easier? Filtering the noise at very low amplitude of 3 to 50 volt post rectification or your AC mains that peaks to 338 volts in 240 volt countries? Don't you think if it was cheaper and easier to filter the AC, the audio equipment maker would put that in the box they sell you? Filter components rated for such high voltages are extremely large and expensive compared to their counterparts in power supplies of audio gear.
And remember, even pure AC at 50 and 60 Hz will still create spikes at 2X their frequencies post rectification. And those spikes would be far higher in amplitude because it is the primary tone:
To the extent you clean up the "wanted" signal above, don't you think you have by far cleaned up the harmonics of the "unwanted" as well?
Really, you are wasting our time trying to create doubt here. This dog don't hunt. Go and ask manufacturers to show you data to prove your points.