I just want to add that I have the true sub MK IV mated to B&W Nautilus 805 currrently. The sub can be muddy and slow w/ excessive overhang.
Luckily, I found the cure by putting it on a lovan jazz 6" stand. The transformation is almost night and day as the issues w/ speed, cleanliness, overhang cease to exist. Now, the blending is about seamless and unobtrusive. I cannot recommend the stand more highly. Have not tried other brands/stands but I am sure the effect is comparable. I believe the reduction in the nasty characters above or improvement brought to the whole musical presentation is even more pronounced than careful stand matching for your main speakers. Those low-frequency rumbling of a subwoofer requires more taming than a regular set of speakers. Plus, when its associated problems isolated and corrected, your speakers will sound even more musical, dynamic and clean.