[particlewave]

This is just speculation based on what I see; the equalization chambers should help prevent drone by introducing the opposing exhausts to each other, canceling out a lot of noise since they should be somewhat in phase with each other.
Obviously, exhaust gasses are a push-push system (as opposed to push-pull of sound waves), but I think phase canceling would still apply to a point.

In other words, the more equalization chambers that you bypass, the more annoying drone you will get. I think this mod (allowing some bypass of chamber 3) is about as far as the stock muffler can go without getting drone/resonance.