Everbody talks about light moving mass but many speaker manufaturers modify the drivers they use by adding damping material. An older Dalquast(sp) Moniter had some sort of putty on the front of the woofer. someone was marketing a doping mod for Mission 707s. One of the most highly rated speakers today use a "damping ring"(my term) applied to the back of magnesium mid-woofers.
Have you tried "doping" your speaker cones?
I recently "doped" my cones. I had read several discussions on it, on the Single Driver Website. It seems Damar varnish is the "dope" of choice. I couldn't find that brand so I used another. I thought it made a fantastic difference. The cones are much stiffer and there is better definition and smoother high end. Bass tightened up too. Had to make a slight adjustment in the "stuffing" in my transmission line tube to cut the bass back a little. They recommend two coats, so that's what I did. Anyone else tried it?