Below are a few quotes from a couple sources that trouble me (emphasis is mine). These little complaints are magnified on my system a thousand fold.
HDCD encodes the equivalent of 20 bits worth of data in a 16-bit digital audio signal by using custom dithering, audio filters, and some reversible amplitude and gain encoding; Peak Extend, which is a reversible soft limiter and Low Level Range Extend, which is a reversible gain on low-level signals. There is thus a benefit at the expense of a VERY MINOR INCREASE IN NOISE.
...in itself, the use of the first bit in the dithered least significant bit stream will DEGRADE the sound quality on a non-HDCD player by decreasing the signal-to-noise ratio but only by a minuscule amount.
HDCD Peak Extension, if chosen in HDCD mastering, will apply compression to the peaks which will be audible in playback on a non-HDCD system which does not apply the appropriate expansion curve.
First, one problem is that if you use a CD player without HDCD (like most do), you will hurt the soundquality. You will lose 1 bit of the 16 bit on the CD and the music can be compressed up to 11,5 dB