It also depends on the output of the speakers. If one gives a lot more than the other (more than 1.5 db), then the top bottom pattern works better imho. The X pattern then favours the higher output speaker and you don't hear the other as well. The G12H/Vintage 30 works really well in an X because both have a 100dB sensitivity. Putting a G12M/G12H works better with the G12M on top because they have a 98dB sensitivity (or 96dB for the hertitage).