The main problem with DRM is nobody can buy or build a receiver that complies fully with the current standard.
It must have both DRM30 and DRM+
It must have all codecs and rates including xHEAAC
The modulation doesnt matter much between 64QUAM and 16QUAM on shortwave. I did enough practical on air tests to show there was maybe a half dB in it. Once your reception gets in the noise below minimum thats it. When things are this bad, only AM works.
The BBC tests on MW went to lower constellation at night due to adjacent channel interference. This is no longer a problem on SW because most everybody has left.