My experience is the same as others here have described -- I downloaded Monster from HDTracks a few days ago and instantly was struck by the awful sounding mix with layers of audio obviously missing or diminished. The problem is common to all 12 tracks. Michael Stipe's vocals, and even backing vocals, are far more emphasized than on the redbook version, which I own, and the crunch and shimmer of guitars and other effects are missing or diminished. It's actually kind of fun to listen to because it's like having a clearer window into the words being sung, breaking through the wall-of-sound to a large degree, but it's enjoyable purely as a novelty. The quality of the audio sounds like absolute trash to me, and I'm normally pretty forgiving. As others have observed, the other 24/96 R.E.M. downloads from HDTracks all sound great, simply higher quality versions of the same redbook stereo mixes. Monster absolutely should be pulled until the true stereo mix is made available.
I should say that, notwithstanding this debacle with Monster, I've purchased a large number of 24/96 albums from HDTracks over the last few years and never had another complaint.
