What do you take when demoing new gear?

CD and LPs.

My ears and my common sense and an idea of what I want and as much information as I can find about it beforehand. 

I usually just bring source material.  However, if I find something I'm interested in, I will attempt to borrow the piece of equipment over a weekend, or whatever period the dealer is comfortable with.

Nothing beats an audition in my own listening room.

Source material, pshaw! First time I saw a turntable at Definitive was unexpected, asked if they had something to play? We dug through a small stack of utterly unrecognizable worn dirty second hand beat up crap until giving up I said this'll do. Whatever it was I do not recall. Not in the slightest. What I do recall is it made the little system crammed up against the wall sound twice as wide three times as deep and five times as good. It was the most engaging system I had heard at that point in time.  

Next the salesman put a CD of Janis Ian Breaking Silence on their top shelf Levinson/Wilson mega system. This was in the same room so nix that excuse. It was loud and powerful and nowhere near as good as the junk vinyl on the Linn. I made up my mind then and there that I needed a turntable.  

If you're seriously thinking of spending money on something so infinitesimally microscopically different you can only hear it in a 3 second snippet of one certain recording, its just not worth it. You definitely need to keep looking. For another component, at the very least.

If you're seriously thinking of spending money on something so infinitesimally microscopically different you can only hear it in a 3 second snippet of one certain recording, its just not worth it. You definitely need to keep looking. For another component, at the very least.

Agreed - no sense in hurting yourself to justify any changes. If it's not apparent, then there's your answer.