The ideal situation has one setting the subwoofer to be coming in at the lowest possible frequency that works to provide the needed support. At higher frequencies, the subwoofer is working closer to the point where it is out of its element and your ear might be confused by the conflicting localization cues (the lower the frequency, the lesser your ability to localize the source of the sound). But, if, for example, you have a suck out at some frequency between say 150 and 200 hz because of floor-bounce cancellation of your woofers on the main speakers (a very common issue), the subwoofer might be filling in for that problem. To the extent the subwoofer can cure problems higher up in frequency than it is normally intended to help, I say stick with the higher crossover. It is MUCH more important to cure problems in the lower midrange than to have ideal subwoofer support at lower frequencies--the midrange takes priority.
Sub best at high crossover?
I am wondering, is my ears deceiving me, or is the Velodyne DD-18 actually sounding better, when I defeat the crossover (by setting it to max, 199hz), and turn the volume some steps down? This is in a fairly large room, with the sub well positioned, augmenting full range speakers. I hope others will share experience.