Do analog audio recordings really sound "warmer" than digital ones?:

Several years ago, a roomful of engineers performed a series of blind tests on a stack of reference recordings to see if they could distinguish those made using tube technology from others made with transistors. All imagined they would have little trouble picking the tube-based recordings by the warmth of the sound. What they found instead was an ear-opener...

Apparently, it's all about the even-numbered harmonics.