I used a dynamat-like product on the mid and woofer in my dads Technics speakers, as well as a similar norez product for the cabinet and that alone has really helped tighten the bass response, and reduce the ringing of the frames at higher volumes.
I still need to make braces, but I'm not in a position to get the wood i need and then travel to my mothers house to cut them.
Tho I'm sure once I can finish that, and add the last of the norez, that should really clean and stiffen things up further.