Hey Scott, I doubt the broadband panels created a peak; they probably created a valley that made other frequencies look like a peak. I know that in general, with concrete, thicker traps are recommended as the walls reflect more LF than drywall for example.
It's good that you got things evened out in there as I'm sure there is lots of ringing going on.

You did such a great job on the omniffusor though, I'm thinking you could build a couple more and try them behind your mix position, or on the ceiling of your tracking room.