You're a big help. I looked at my measurement setup, and EUREKA, the path diff. between the lower and upper woofs to the mike was...13 or 14 inches. Right in a place where dual slit diffraction would cause a big null. When I tested the 4648a-8 by itself, mike was at the same height as the upper woof, for a path length diff. around 9 inches, that and the woofs being 16.7 inches center to center both are implicated in the 817 dip...but I never looked even higher...measured on axis with the upper woof the system was measuring +/-1dB from 100 to 1000 except for the 817 Hz. cancellation.

I've now listened with fixed 12 dB pads on both systems, one set x'd at 600, the other at 1600 does sound a bit thinner, so the level dip from 500 to 1 k is obviously audible. Guess I'll go back down with them. Sam