I'm running my 604's at a crossover point of 1150.

[cradeldorf]

I've read that 604 E to the last sectoral before the CD types were only providing driver loading to about 1900 Hz but the Altec standard Xover is 1600 Hz

I could never grasp the concept of loading since the phragm is barely moving in the first place and then every bit of air it is moving has to pass through 3 little human hair sized slits in the phase plug?
I can't imagine I could load the horn if I put my lips on the phase plug and blew as hard as I could. That is if my definition of loading means building some kind of back pressure from the air trapped in the horn tube and horn assembly itself. I would surely think the phase plug is what provides loading if there is to be any.