[DrWho]

I think I would start by turning down the subwoofer ~10dB so that it's output is more in line with that of your mains. This will push the interaction point of the relative phase between your mains and sub closer to 60Hz, which is where you've got some nasty modes happening. In other words, right now you're running an electrical 60Hz crossover, but are seeing an acoustic crossover of 75Hz.

Another alternative would be to lower the crossover frequency to 50Hz so that you're seeing closer to a 60Hz acoustic crossover. You would probably still need to reduce the output of the sub, again, the goal being to see if your phase switch can help fill that 60Hz null.

If neither of the above approaches work, then your best bet will be to reposition the subwoofer (in fact, you might as well just start with moving the subwoofer). Don't forget the option of moving the listening position too. The position of nulls in the room are mostly the result of the standing waves that build up...moving the speakers around doesn't change the dimensions of the room, which is to say that the standing waves generally want to happen in the same location. For instance, take a measurement in the rear corner of your room and note how different the response is...