Also. I put mortite on the centerpost, handles, baffle, sides of baffle..anywhere i could remove or dissassemble the cab, i did and sealed it with mortite. The ampeg cab was worse because it had more screws, but literally, its just time and labor. The mortite is $6
Just for your fyi, I literally chased EVERY solution with my cabs mentioned in this post.
My Dad and I solved this and were stunned at how much tighter, and better my cabs sounded at stage volume. And for his part "he hated music, cant stand this guitar shit, get a job" but he ALWAYS helped.
So as much as a non guitar nerd opinion may not sit well with us tonally, it pays to have a non musical ear helping and confirming and adding non guitarist logic. Later, he helped me re-eq my rig so it didnt bother his ears at volume (no high end or bass, all mids, i hated it, i wanted scooped). But all the sudden I was "clearer louder" and sounded "better" than last gig. Dude knew his stuff for someone that hated music but created a complete guitar/amp gear freak in me.
Ill note as well, my rig was solid state mangling the sonic space with bass so theres that..i think that rig could have rattled any cab loose though.
Even worse how they sometimes do the "after note" vibrate a little at low volume and u go "wtf..oh well i can live with it".. then on stage fart city blah.
now that im all tube my cabs are dead silent because that square 11th order odd harmonic or whatever the f isnt bashing my cabinet's centerpost cervix with solid state 65hz.
Also; not all cabs will benefit from this. I did it with my Rivera dual theile q212 and couldnt get it and the poly fill back off.
Total pain in ass to unscrew and remove, i was pisssseddd! Haha