Your article about the Lightspeed PFX ANR headset in the August 2014 issue of Aviation Consumer could not have arrived at a more opportune time and was spot on in its evaluation.
I had just purchased the Zulu PFX a few days earlier and while I think it is the quietest and most comfortable headset on the market, some of the same bizarre effects that you noticed—low-pitched fluttering and ANR performance issues when moving the head—were present in mine as well. I have serial number 189.
I did reach out to Lightspeed and they thought it may have been a fitting issue. The company immediately sent out new earseals and a larger headpiece cushion. I changed both of these and noticed that the ill effects lessened considerably.