I think it depends, as others have alluded, on the ultimate source material. I made the assumption that unless it is a Chesky release, HDTracks has merely contracted to but the downloads from the distributor/source company. I have limited my purchases to those where the studio has remastered and released a work as a hi res release on DVD-A or SACD. I assumed ( and I am sure in some cases incorrectly, but reduces the risk, from a logic perspective) that the hi res remaster was used for the FLAC download.
Also, HD tracks frequently has 10, 15 and 20% off sales, so I never pay full price. I also focus on those recordings that are hard to find in CD format. Lots of jazz, classical, etc. I also, with a few exceptions for comparison purposes, have not replicated anything I own on CD. The only exceptions were in a few cases where I had the CD, DVD-A version. I wanted to compare each source. In many cases I was also able to compare a ripped DVD-A with ripped 16/44 CD and 24/96 FLAC.
On headphones, in some cases, I could definitely tell the difference between CD and FLAC or DVD-A. Much harder to tell between ripped DVD -A and download FLaC. The one I found to be the test that was quite startling was some Talking hEads, where I have the CD, DVD-A ( both dual disk and British DVD-A standalone). The hi Rez versions were indistuishable between download FLAC and ripped DVD-a. CD sounded worse to my ear. Some CD's, like Plant/Krauss "Raising Sand" where the CD sounds pretty darn good, and it is mainly acoustic driven, so likely little above 10 or 12,000 Hz, so I felt benefit of the hi res would be limited.
So far I am pretty happy with the strategy.
However, they do charge a premium price for their product, and in many cases, they include cover art, but no liner notes, which is a value drop vs the Cd, IMo. To compensate, I would like to see some information on the origin/source of the files being downloaded. They are likely selling to enthusiast who care about this stuff, an effort to do so might result in more sales.