[brucek]

I was surprised to see a half-way decent before graph

And you're doing yourself a disservice by using such an expanded vertical scale.

If you used the more reasonable standard that we use here of 45dB-105dB, you'd be amazed how flat that response actually is. You're using about a 20dB spread rather than the accepted 60dB. You should also take your measurements at 75dBSPL.

brucek