While it is good and may seem logical to have your 8-9 prime demo cuts you always go to when evaluating a new component, it is also fraut with danger - "familiarity breeds contempt".
We humans can have a tendancy with very familar material to all ready have a " preferred sound" we are evaluating in our heads. We begin to somewhat stop listening to the music and listen to our pre conceived "preferred sound" checklist. As Lew said, you then play less familar/new music and the results are not always as convincing.
I fully agree with Lew. You need to live with a new component and play LOTS of different music over a reasonable period of time, then go back to see if it really was an improvement or just different.
Of course, like Siniy said - there is no perfect cartridge for all music genres or recordinds within a genre - but sweepeing statements that MM is good for ex and MC's are better for y is also not correct.
And, I feel a lot better within myself that my Technics EPC100C is back as equal " best cartrigde" :-)
cheers