It might be added to that while mastering over format is the biggest issue dictating sound quality (no arguments here) --- it should be noted that the hardware utilized in the playback matters as well --- there are some really good redbook CD players that can, on average, make redbook CD's in general sound better than a modest SACD player can make the typical SACD sound.
Some of the newer redbook CD players are astoundingly good.
Some of the newer redbook CD players are astoundingly good.