Three years ago I replaced the Altec 811 horns with Altec 32B horns in my 2-way speaker system. I think I now have an unscientific explanation as to why I've been enjoying the Altec 32B.
Somewhere along the way I acquired a pair of RCF H3709 horns. I was rooting for these horns because they resemble the Altec 32B but with a straight throat. The RCF H3709 to a much lesser degree, still exhibited some congestion and harshness I found objectionable in the Altec 811 in near field listening. Perhaps a judicious amount of damping applied to the thin walls of the RCF H3709 will significantly improve its performance.
Based on the throat comparison picture above, I attribute the sonic superiority of the Altec 32B in my listening environment to its unconstricted and constant flare throat. But that's just my empirical observation ;)
Here's a quote from a post by Steve Schell at the Altec Lansing Forum
Joe Roberts had nice things to say about the Altec 32B at the High Efficiency Speaker Asylum and Audiokarma.