Hi I don't think there is any one best location, other than the one the designer used to develop the crossover. Here are two measurements of a center channel I'm working on. In the first, the speaker is reseting horizontally on another speaker cabinet to simulate a placement on a TV or in a shelf with a cabinet front below it. In the second, the speaker has been moved out so that there is no baffle directly below it, to simulate a placement on a shelf with nothing below it. Both plots show a dip in the bass, which is due to floor bounce cancellation and will be present on any speaker unless the woofer is right next to the floor. And both show a little peak in the 70-80 hz region which will be present in most any room. But the free standing speaker clearly has less bass, and an accentuated midrange, which will be clearly audible. I suspect most people will have something underneath the center, so that's probably the best crossover tuning. But ideally you would want a different crossover for each situation.