EXPLAIN HD SACDs to me.

if i glean correctly, your central question is whether hdcd/sacd are superior than redbook cd. hdcd does contain a little more musical information and has more dynamic range; all things being equal an hdcd should sound richer and better than a redbook cd, though not night-and-day better--hdcd is still just an enhanced redbook cd at core. presumably because the difference between hdcd/redbook is comparatively small, the hdcd format doesn't get a great deal of attention. sacd, on the other hand, is an entirely different beast--it utilizes much higher sampling frequency and provides much greater resolution and dynamic range, as well as much greater disk capacity, multi-channel capability, etc. all things being equal, sacd ought to sound vastly better than hdcd or redbook. however (and there's always a however) there have been a number of blind listening tests in which subjects (including recording engineers) could not discern an audible difference between sacd and regular cds, conceivably because most people can't distinguish audio over a certain frequency threshold. interesting stuff.