That "sound" is partially due to the design of the current AM system. It was built to pack more transmitters into the allocated frequency range.
There are no technical impediments to high fidelity audio because of amplitude modulation. In the past the big limitation was the bandwidth of intermediate frequency (IF) amplifiers in the receivers which were typically tuned for maximum gain and not flat audio band response. The transmitters have flat response out to the legal limits. Radio stations have a monitor receiver that has proper IF audio response and the sound of those are very good.
That's he-aac so 56kpbs is not that bad, it gets dicey below 32kbps when you need tricks like parametric stereo. But he-aac's spectral band replication does add metallic taste to everything, i would agree
There are no technical impediments to high fidelity audio because of amplitude modulation. In the past the big limitation was the bandwidth of intermediate frequency (IF) amplifiers in the receivers which were typically tuned for maximum gain and not flat audio band response. The transmitters have flat response out to the legal limits. Radio stations have a monitor receiver that has proper IF audio response and the sound of those are very good.