You need to distinguish "distortion" which the ear likes vs. the kind it doesn't. The ear likes octaves, fifths, etc. I googled it for you and came up with this.
audible distortion and tube gear?
I know that I have read on this site about "tube sound" having something to do with the way tube gear handles distortion, but would tube gear make it harder to recognize distortion?
The reason I ask is because I have a test CD that has several consecutive tracks of a test tone with no distortion and then on each track distortion is increased by a certain percentage. I am not sure I am hearing what it says I am supposed to be hearing as the distortion increases.