I never heard the 511E but if it is similar to the 511B, and I think it might be, that could be your problem right there.

In my estimation, none of the Altec cast aluminum horns are as good as the drivers. Some people like them but I could never make them work. The 511b particularly strikes my ear as rather harsh.

Are you using the 802s on 511s also?

I had 311-90, 31A, 32B, 349A, 511, 811...junked them all. Now retrying the 32B.

I'd say try a good horn...multicell or maybe even a modern creation like a LeCleach. Any 288 in proper working order should be capable of refined smooth sound--maybe even smoother than the 802 (but not as extended on HF).

Maybe you need to clean the gaps on the drivers?