I think I might be onto something. I found a diagram with calculator for a
twin-t notch filter on the Okawa Electric Design webpage. After punching in resistances for a while, I figured out what I needed to notch out 11. Went, got the parts and built a prototype on a bread board. So far, 11 has been attenuated considerably, 9 isn't acting up and I get a weak signal from 10, which never showed up before. I have yet to try it where it should go, between the antenna and the pre-amp.