[JohnM]

The slight ripple you see at the low end is caused by a narrower impulse response window than is needed. If you look at the Impulse Response graph you'll see a section of the response that is not being included, so extend the window duration to cover all the area above the noise floor and the low end should show a smoother curve. If you post a plot of the impulse response (with the Y axis set to dB) I can indicate exactly what should be included.