Neutron notch filter, how?

[base13]

HI there,

I'm trying to build a Neutron using RG Keen's schematic found on GGG. Everything seems to work fine, except for the extra "range" addition, the notch filter.
It is supposed to work by mixing HP and LP right? Well I tried buffering the HP and LP filter parts using an 4558, this does give some output but very distorted. HP, LP and BP still works fine.

What am I doing wrong here? Anybody build a Neutron with this feature and got it working? Please tell me how you did it.

Thanks!