You know I love filters. You also know that the Lowther measurements on my site clearly show the peak you describe. But for this particular peak, I would probably not use a filter. All you need to do is listen about 10 degrees off axis and the response is nice and flat and does not start to roll - off too bad at higher frequencies. A second option is a phase plug, there is a site linked ftom mine that shows Lowther DX3 measurements with several different styles of phase plugs. I have a growing collection of Lowther phase plugs and can state that they can be used to tame this on axis peak.

Hope that helps,

Martin