EXPLAIN HD SACDs to me.

Bear in mind that not all players that purportedly play both SACD and RBCD do a "proper" job with SACD. Many will convert the SACD datastream into PCM, and then process it through a standard 44.1kHz DAC. You will lose the advantage of SACD and maybe even get sound which is worse than RBCD. To play SACD properly, check if your player has a DSD DAC.

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.

Vernneal, I suspect that the earlier posters are correct that you are really asking about HDCDs. Only the Crystal chip set, I believe, plays HDCDs, which have been somewhat lost with Micro Soft buy this format. The original Oppo uses this chip set and does play both HDCDs, CDs, and SACDs. I have never figured out how to force the Oppo to play HDCDs as CDs, but the HDCDs do sound excellent. Many performers want their recording done on HDCDs.