HDTracks is a website owned by David and Norman Chesky, two well respected names in the audio industry, that sells "hi-rez" music downloads in 24-bit flac format. Typical album prices are in the $18-$20 range, with many costing more than that. You are paying a premium price compared with purchasing a physical cd, in order to receive what is supposed to be an audiophile recording of higher quality than is available on cd. I have personally purchased many titles from HDTracks that I play on my Squeezebox Touch.
I recently read several articles online discussing the questionable quality of the downloads from HDTracks, with accusations of many titles being mere up-samples of 16-bit recordings. Curious about this, I downloaded a software package called Audacity which is capable of performing various analysis routines on the music files. I was not happy with the results....
Take, for example, the "In Session" recording by Stevie Ray Vaughan and Albert King. Here is what the first track looks like with clipping detection enabled (the red lines indicate clipping). This recording appears to be highly compressed, precisely opposite what one would expect to find in a true 24-bit recording:
Here is a frequency analysis of the same track, showing a complete rolloff at ~22khz:
I have noticed similar issues on a large number of the titles that I have purchased from HDtracks. In the articles I read, it did appear as if HDtracks has admitted to some problems existing where upsampled music was accidentally sold, but that steps would be taken to ensure this did not happen again.