Wayne has a sure-fire solution, but I might offer a few less ingenious ideas...

If the iron is triggering the problem (as clearly appears to be the case), you are getting noise on the circuit from the iron's electric heating element. The reason that turning off the LFE doesn't help is that the noise is presumably coming into the sub via the sub amp's power cord. I'd try moving the sub to a different circuit than the outlet used by the iron or some sort of power conditioner for the sub.