Slaw's idea of using an agreed upon recording as a reference for discussion is a great one. As such, I would like to get the ball rolling.
While I am not very familiar with the musical tastes of the contributors to this thread, a popular and commonly mentioned recording on this forum is Donald Fagen's "The Nightfly". While not necessarily my favorite genre (post '50's jazz and any classical are) it is a great example of the kind of the urban-hip sound/witty lyrics sensibility that made Steely Dan so popular. It is a recording that some days (depending on my frame of mind) I think is fantastic, and other days I can't stand to listen to it because of it's slickness. The playing is unquestionably great on all counts. It is also notable (IMO) for being fantastically well recorded and produced, and great sounding for a purely digital (Horrors!) recording. I have the original Warner Brothers pressing as well as the later Mobile Fidelity version and prefer the original, with the MF having a strangely dynamically polite quality.The track "Maxine" is a favorite and there are two musical moments on it that I often use to test how well I have fine-tuned my arm/cartridge set-up. The track features the incomparable and sorely-missed Michael Brecker on tenor saxophone, and both of these musical moments involve his brilliant (as usual) solo. I will mention each of these moments in reverse order, as the second one is more obvious and less subtle.
The tenor saxophone solo begins at 2:29, and ends at 3:02; or so we think. It culminates with two ascending runs followed by one final short statement at 3:02. At 3:03, however, the overdubbed vocal chorus comes back in singing "move up to Manhattan". At that precise moment one's attention is drawn to the vocals following the saxophone solo, and it is easy to not notice that at the very moment that "move..." is sung, the tenor saxophone plays what is in fact the true end of the solo. He plays a one note final commentary, that is a kind of musical exclamation point.
When the VTA and azimuth are not adjusted properly on my ET2 it is easy to not notice that final note; it disappears into the fabric of the vocal chorus. When things are adjusted correctly, that final note is heard clearly and distinctly from the vocals; it gives that solo even more meaning.
The second example occurs at 2:51, also in Brecker's solo. One of the things that made Brecker's sound so distinct was the post-Coltrane technique of "splitting" a note. What is meant by that is that the player is able to play a note and make other notes sound at the same time; gives the sound of that note a very dramatic quality. The way it is accomplished is by (in this case) playing the high G on the tenor by fingering the G an octave below and sounding the harmonic one octave above. In the process, if the player has enough control over the instrument, a third note "E" can be heard; he is in effect playing a chord on a melody instrument.
When tonearm/cartridge settings are not correct, what should be heard as three distinct notes becomes simply distortion in the sound of the note. The closer I get to correct VTA and azimuth, the more distinct the three notes become.
I have always felt that one of the beauties of this arm is the ability to adjust so many parameters easily and repeatably. It can truly get the best out of most cartridges.