I had an OEM mid fail (Jensen Imperial corner horns, ca 1950's), originally spec'd at 600Hz to 4kHz. Diaphragms are no longer available for this driver. I'm all actively crossed over, so figured I had the flexibility to try different horn/driver combo. I bought Altec 511B and Great Plains Audio 909A, but despite fact both horn and driver are spec'd to 500Hz, it was readily apparent this combo was severely lacking below 1kHz. Looking for help on another forum I received a response that included the following:

"Below 1k the 511 doesn't provide enough/proper acoustic load for the 1 inch Altec drivers.
...
Further, the small format ferrite drivers from Altec(which the GPA driver is a close facsimile of, but NOT exact copy) all exhibit a frequency response that is shifted upward nearly an octave from their alnico predecessors...
...
In a nutshell, the newer style ferrite Altec drivers don't belong on a 511....
First choice for a small format mid driver from Altec would be an 808-8A with the original symbiotik diaphragm."

I have very little experience actually using horns (other than 10 or so years with these imperials), and very little knowledge about them to boot. I greatly appreciated this fellow's opinion, and it fit in with my one experience, but wondering if others agree with the assessment.