Helpful information.
With the 28 ou. magnets, the phase plugs are blunt. On the 40 ou. magnets, the phase plugs are bullet shaped.
I've replaced the 28 with the 40 ou. magnet woofers and Brian said there is no difference in the sound. However, there is a difference in the 40 ou. will not get overdriven like the 28 can--the 28 makes a loud blat sound. Not good for the longevity of the woofer. After switching to the stronger magnet, I never had that problem. I listen to several great bass players such as Stanley Clarke, Victor Wooten, Jaco Pastorius, Marcus miller, Brian Bromberg, etc. They will definitely excite the 28 ou. woofers into overload at times.
I've also done the MK II upgrade to the 10" passive on the bottom side of the cabinet and plugging the holes of the old passives after removing them. It is a tedious, and time consuming job but the quality and power of the end result is easily worth the trouble. I no longer have the 30s but have a very fine set of 40s that I love.
Bob