I borrowed one HDCD, but found no difference, though my sound system responds well to the quality recording. Sometimes it is not a bottleneck. No, most often it is not a bottleneck.
Edit: right now listening to Diana Krall's CD I heard a rattle... I brought a signal generator and went from 20 Hz to 20 KHz, but did not hear anything... I put that CD back on the same tune and heard the same rattle again...
It is an ordinary CD, but it fooled me well. It means, it was recorded properly No blue nor ultra-violet rays may help to make bad records better.