Listening has become a test procedure for me, much of the time.

I'm typically listening to learn about the wide variance of frequency response and stereo effects (or the lack thereof, for headphones) in audio systems and their environments. Especially when I'm working on a mix, it's incredibly valuable to try and optimize for every situation. There's many artists' songs that I like to jam on every system that I can, in every context I can, trying to infer the subtle changes in the audio (sometimes I actually succeed). Hah, I have to take care not to be obnoxious about wanting to listen to a calibration track on friends' systems.

I actually don't have a good stereo setup that I have reliable access to. I mix in headphones (Shure SRH840, decent value) and then listen to it on my crappy monitors (name withheld, to spare me the shame), a mono tube speaker (yep), my podphones (yep), my car speakers (bad), and my monitors at work (c'mon, of course they're bad). If it sounds good on all of those, it's pretty likely to sound superb with a great stereo setup.

When I'm casually listening to music, it doesn't matter near as much. Roll the windows down and blast it driving down the highway, it's already summer here. And if I couldn't listen to my music during a workout, well, that wouldn't be a good time.