Cross-bracing should help reduce panel resonance, but can also decrease volume a tad, so keep the brace diameter to a minimum. I'd also consider adding a bit more stuffing (wool, acousti-stuff, polyfill, or dense memory foam) to help absorb internal resonance. You can it in increments until you like it. Keep it away from the woofer cone, and port area. No harm done, and it's un-doable if you don't like it.
Upgrading the crossover components and wires can be fairly inexpensive, and can set the stage for better clarity in your system. I'd change the iron core inductors to heavier gauge air core, and change the cheap electrolytic caps to a poly film cap of approximately the same value. You could also add a very high quality small value bypass cap of something like 0.01uf to help improve treble. A lot of folks will say it won't make a difference....each change can be tough to detect by itself, but all totaled, if your system is good, it can be quite audible. One thing is for sure, if you leave what's there installed, it'll likely always be the weakest link in your system.
I suspect the extra magnet is a bucking magnet for shielding.
In the end, it's hard to say if these really worth much of a cash outlay or effort, but it could be a good learning experience for not a bunch of money.
