Ok, so filters will indeed create an an electrical phase shift at the crossover frequency. If the slope is 24db/oct the shift is 180 degrees, 12 db is 90 degrees. But normally there are two filters involved and the effect is cumulative. So while a 24db LPF on the sub creates a 180 deg shift, a matching 24db HPF on the underseats would also be 180 degrees, so the total effect would be 360 degrees, which would again mean zero phase shift. If the two filters were 12 db, then the phase shift would be 90 + 90 = 180 degrees. So, if the subwoofer and the mid bass were the exact same distance from the listener, then the 360 degree shift would result in no cancellations, while the 180 degree shift would result in massive cancellations (at the crossover point). However, the mid bass and sub are usually NOT the same distance from the listener. The difference in distance creates an acoustical phase shift. To remedy any potential cancellations at the listening position, due to electrical and/or acoustical phase shifts, one can simply try to reverse the polarity of the sub. This is explained in my "Scooby Doo" thread.
http://www.e90post.com/forums/showthread.php?t=748569