A much more likely explanation, I would think, is that low level broadband noise (i.e., hiss) in the upper treble region is being reduced, and that is being subjectively perceived as a diminution of the highs.
I've seen it stated in a number of articles that I've read in the past that low level high frequency hiss tends to be subjectively perceived as "air" and ambience. Which would seem to make sense.
Almarg (System | Threads | Answers | This Thread)
I've heard about this from several sources also. One problem I have with this is the fact I can still hear this hiss in the music I transferred to a CD. With this same music on the CD, it seems to have lost the those high frequencies, some of us refer to as air. Possibly, there may be a higher frequency hiss that we don't actually perceive, but may be there on analog, and cut off by the filter in the D/A converter.