The mp3 preference is an old phenomenon in a new package. There are still plenty of tube enthusiasts out there who prefer the "warm" sound of tube amplifiers over solid state electronics. Tube-based microphone preamplifiers are still quite common. It turns out that the "warm" tube sound does not have flat frequency response -- the warmness is because of peaks at certain frequencies. So you see, people over 50 who prefer tubes have the same issue, if it is an issue, as people under 25 who prefer mp3's.