Its all about the rules of human hearing. Negative feedback generates distortion that the human ear is very sensitive to: odd ordered harmonics. They cannot be eliminated unless you also get rid of negative feedback! So amps that are zero feedback will often sound inherently smoother and more musical than those that use it. more:
http://www.atma-sphere.com/papers/paradigm_paper2.html
http://www.atma-sphere.com/papers/paradigm_paper2.html