So what form did the notch filter take? Did it plug in to the audio output or was it professionally fitted much farther back in the receiver? If you have interference on audio a notch filter is of little use unless the interference is a single tone or some sort. I suspect your improved receive performance was derived more from the outdoor antenna and good co-ax. However, a simple vertical dipole would work as well, if not better, than an expensive discone, which is trying to be a "jack of all trades".