I have a couple of humbucker/humbucker guitars that have that quack in spades when you play both humbuckers together. They are really intense and some other H H guitars have the quack nuance but not as intense. Think of the old Beatles recording Honey Don't. George Harrison has a great tone there and that quack is sounding awesome. I can even get that on a few Strats combining mid/bridge or neck/mid or even neck/bridge on the one that has multi switching combinations. Reason why I started wondering is due to the Les Paul I was messing with tonight. I have a Duncan in the bridge and an old DiMarzio in the neck. But I am noticing it has no quack at all playing both pickups together. Don't get this quack I call confused with the out of phase/phase reversal thing. I started wondering if it is due to how I orientated the coils north/south on the series connection junction on the neck pup. I have a DPDT switch on the neck that goes from series to parallel within that neck pup. I had a problem with it as a wire was grounding out inside and caused the series position to actually be single coil setup. Oddly I noticed it had some quack in single coil mode combined with the bridge pup, but then after I fixed it the quack was gone. So I started looking at DiMarzio wiring diagrams and it seems I an using a different config on connection the north/south coils orientation. Has me wondering if that is why the quack is non existent? Then I have always wondered what happens if you take a pickup out and flip it 180 degrees - does that cause it to now be out of phase with the other pickup(s)? Or could that cause or enhance that quack tone?