i could be wrong but i think somewhere around 100-120hz frequencies start to get directional you are also getting into the 'midbass' around there,so,you x-over at 250 hz,you start to lose SOME 'imaging', but the biggest thing i've found with many subs(home and car) is :

they don't like to reproduce midbass,the sound turns muddy,it gets really overbearing,and it robs the ever so important power to do what subs do:reproduce low frequencies.

i don't know what advice to give you other than i would never x-over a sub that high.