Well, I've been fiddling around trying to see if I can get just the upper sections (no aux speaker) to flatten out a bit. According to
this site I used a .8 mH inductor and a 20 ohm resistor to make a BSC to flatten out the high end. The results were less then spectacular.
Oh well. Back to trying to tame the tweeters. Actually - that's not a fair statement. The tweeters sound fine, they just don't seem to measure well. I'm trying to verify that the problem is in the measurement equipment (where I think it is).