Hereís an answer Iíve been kicking around: Your system is becoming more neutral whenever you change a system element (component, cable, room treatment, etc.) and you get the following results:
(1) Individual pieces of music sound more unique.
(2) Your music collection sounds more diverse.
This theory occurred to me one day when I changed amps and noticed that the timbres of instruments were suddenly more distinct from one another. With the old amp, all instruments seemed to have a common harmonic element (the signature of the amp?!). With the new amp, individual instrument timbres sounded more unique and the range of instrument timbres sounded more diverse. I went on to notice that whole songs (and even whole albums) sounded more unique, and that my music collection, taken as a whole, sounded more diverse.
That led me to the following idea: If, after changing a system element, (1) individual pieces of music sound more unique, and (2) your music collection sounds more diverse, then your system is contributing less of its own signature to the music. And less signature means more neutral.
P.S. This is only a way of judging the relative neutrality of a system. Judging the absolute neutrality of a system is a philosophical question for another day.
P.P.S. I donít believe a systemís signature can be reduced to zero. But it doesnít follow from that that differences in neutrality do not exist.
P.P.P.S. Iím not suggesting that neutrality is the most important goal in building an audio system, but in my experience, the changes that have resulted in greater neutrality (using the standard above) have also been the changes that resulted in more musical enjoyment.