I'm probably a weirdo about this, but I tend to "test"(initial listening) transducers with messy stuff that can easily be pushed too far for my own taste, or can on the contrary sound overly cozy even though they're definitely not if played on a clean calibrated system. Even then, it alerts me of a massively weird frequency response and not much else.
I go for the mess within the mess of one of theses usually:
It's not hard to get some punk or grunge stuff, with distortions everywhere and confused mixing, I just happen to know and like those tracks.
Otherwise, the tracks always used to pretend like I'm getting an idea about the sound:
-Pastime Paradise (all, with intro in particular).
-November Rain (the end around 6:50mn with the guitars having some fun) I can do a pretty good job of EQing with this one alone, just getting the guitars on both sides with a somewhat similar placement and loudness helps me a lot, and the cavernous voice mix thingy also changes a lot with EQ in the right places (could just be force of habit and nothing particular about that track, IDK).
When I started "testing" audio gear, I would go for the usual vocal goddesses and classical stuff you could find on all the demo discs out there. Never helped me. The rare occasions where I could argue it did, were for big room/speaker issues in the low end, but plenty or rap tracks made the issues just as clear.
Great tracks tend to sound nice to me, even from the PA system of a supermarket, so I've stopped using those demos pretty early. It's the stuff that's just a step away from sounding horrible that helped me the most in practice. That, and of course knowing the track very well from a calibrated speaker setup.
I'd love to pretend that Mezzanine, Fast Car and the Toccata told me everything about gears, but beside being hyper used to them, they never revealed anything particular to me, and when they did, other songs did too. I could never use Pidgeon or Krall on the demos, I don't like them for some reason, taste is a strange thing (my nemesis is Celine Dion). So, for women voices I'd go with Winehouse, Sade, or even Tori Amos sometimes. Not sure if it helped me at all, but at least I did like listening to those girls and always felt that it was pretty important to like what we use.
A manual switch and direct comparison has helped me so much more than a particular set of tracks.