I've had a few pairs of 511B and 811B horns. I can tell you with a couple of things with certainty based on empirical evidence:
*the all-welded 511B has significantly more ring than the all-welded 811B
*Either all-welded horn is damped considerably when baffle mounted or mounted on the Altec "right-angle" wood mount that was typically used atop the A7
*the late-production 511B with silastic bonded sectoral webs vs. the earlier welded webs is quite dead enough for me, especially when screwed down to a baffle or A7 mount.All my experience with these horns confirms Wayne's contention that the best way to damp these horns is to de-stress them by parting the sectoral webs (cut the welds). I really don't know how to do that neatly with hand tools, but if I couldn't buy a pair of late production horns, I'd take welded horns to a local machine shop and pay them a few dollars to part the webs for me.