That was a clever idea to use a notch filter to deal with the bass ripple of the undersized box.
I find that modelling programs tend to underestimate the bass ripple of sealed enclosures. Since I have a good in wall sub with the driver right under the center speaker, I have dealt with mine by raising the crossover to the sub. That worked perfectly as in my case the ripple was from 50 to 100 Hz. Although I did not use undersized boxes, but sealed enclosures are so prone to that ripple. My theater surrounds have ripple as well, giving some warmth.
That was a good dodge dealing with a notch filter.