To go into a bit more detail, the cone-rocking problem in the Thunderchild 112 prototype arose from a combination of factors. First, the ports were way off to one side of the woofer cone, resulting in a severely asymmetrical air load on the back of the cone at some frequencies (it only happened on some notes, probably related to the suspension system's behavior as well)). Second, the long voice coil of a Kappalite-class woofer doesn't need to be rocked at much of an angle before it taps against the side of the magnetic gap (which is the cylindrical hole through the middle of the magnet where the voice coil resides). Third, the Thunderchild 112 was a smallish box for that woofer, which can handle a LOT of watts, so the result is that it's a very high-internal-pressure box. Those higher pressures exaggerate the effect of the asymmetrical airload.
So after tearing my hair out for several days I called Jerry McNutt and told him about the clicking. He asked me to describe the box configuration, diagnosed the problem, and suggested moving the two ports down so they were symmetrically positioned behind the cone. He suggested placing one port above the other, so that we'd get a little bit of "chimney effect" cooling, as hot air would tend to move out of the upper port, drawing cooler air into the lower port. Fortunately there was just barely enough cabinet depth for the ports + woofer magnet, but the unbraced area of my back panel (there's a horizontal window-pane brace in between woofer and horn) was too small for me to stack the ports vertically (they're flared ports with large diameter outer flanges). So Jerry said to put them on a diagonal, and they fit that way, so that's what I did!
That cone rocking/clicking thing was really traumatic for me because, until I had it solved, my super-cool first bass cab (which was pretty much my only hope to save my company from the Great Recession) had a totally mysterious and totally fatal flaw. So I probably over-reacted to it.
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Back to ports for a moment, I've gotten a lot less paranoid about cone rocking because nobody has reported it since I made that change, so the 1553 will have three ports in a clover-leaf pattern centered behind the woofer cone. Plug one, two, or all three... but when you just plug one, maintain the vertical separation. You'd think chuffing would set in as we severely reduce the port cross-sectional area below optimum, but remember we're dropping the tuning frequency at the same time, down into a region where there's not as much energy so we can get away with the reduced port area on a bass cab.
