I hate to wade in with more advice, but:
1. If you take a recording (see 2 below), I suggest that you don't take mp3's. The frequency response is limited. Record wave files of PianoTeq. I would also suggest taking a cd. Many pro shops might not be set up to let you insert a flash stick into their computer.
2. The best thing might be to instead take your laptop (if you use one) with your external sound card and usb to midi cable. The shop could then very easily plug monitors into your external sound card and a keyboard into your computer, so you could actually play PianoTeq over the monitors. Just ask the shop--you might want to drop by beforehand to be sure that this is ok, and to ask when the best time would be--when it's not crowded, for example. Most shops won't consider this request unusual, now. This way, you could also listen to all of the presets, and experiment with various parameter changes. (I wouldn't worry about the amount of effort you're asking them to expend--if the shop is a good shop, they expect to spend time on people shopping for good monitors. It's not a small purchase, and the only way to hear if a monitor is good for a given sound is to play that sound through it. If the salesperson seems impatient or too burdened by your request, go someplace else.) Also: find the piano player in the shop. He or she will save you a lot of time and will also be interested in hearing PianoTeq through different monitors.
3. Don't rely on JUST the size of the cones, comparing just small ones to just one or two sets of large monitors. Listen instead to several monitors with large cones. Some will sound better for a piano than others.
Let us know what you find.