I'll put it this way, if a station does a good job with audio processing and making sure the entire audio chain and transmitter are in great shape, a C-Quam AM stereo station can sound very good on an AMAX radio. I know of one that still does this, WION in Ionia, Michigan. They actually set up their streaming by feeding it with the sound from an AM stereo tuner. It's a gimmick, but it sounds very good. You can hear it on i1430.com if you want.
All that being said, it does not sound as good an an FM station. I suppose an average listener might not notice the difference. Even this station that really hypes their AM Stereo signal has an FM translator. But I give them credit for doing a really nice job with their AM signal.
The all-digital signal in DC is an interesting experiment and I would love to hear how it sounds on an HD radio. However, it's not going to sound quite as good as an FM station.
You're just not going to get a better sound out of a signal that is squeezed through a much narrower bandwidth.