unless your wiring is messed up and some polarity is reversed there should be no "phase cancellation, " as this only happens when two frequencies of equal amplitude are 180* out of phase. phase discrepancies that you hear regularly from a 4x12 have to do with the physical distance between your acoustic receiver (ears, microphone, etc) and each sound source, because you have 4 sources and 3 dimensions of space which takes time to traverse. these are small, but very noticeable, cancellations. if you were to be equidistant from each driver and each driver were the same, you would not hear this. (although you would still hear phase in the room, but thats how we tell directionality so don't sweat that right now).
because you're doing an X, if you're very close to your rig i am sure there are inherent phase discrepancies in how each speaker handles voltage, one of the big reasons we choose one speaker over another. however, this should still not give you a "the top is louder result."
if you position yourself straight on with the cab, so that your head is close to the center of the throw, about a meter or two away, and the top still seems louder than I would have to agree with the post above that you have an impedance issues and would question your wiring. remember, 2-8ohm loads in series is 16ohms but in parallel its 4ohms.
let us know what you come up with,,
