IIR Cascaded Filter Response, Unexpected Results
Looks like a simple sign error in the denominator. If you use a = [1 -1.866685 .91058], you get something that looks notch-like at around 380 Hz.


  The Form is Direct Form II


This is numerically the worst. Shouldn't matter here, but it's good practice to either stick with Direct Form I or Transposed Form II.