[JohnM]

Macca,

Click the IR Windows button in the REW toolbar then adjust the Right Window value until the dotted line is out where the impulse response dispappears into the noise level. For the blue trace that is about 650ms for the response you captured (but see the noise floor comment that follows), for the red trace about 1200ms. After you adjust the value, hit the Apply Windows button to apply it. You will then find the responses roll off more smoothly at low frequencies.

The noise floor is high for the sub (blue) trace because you did a full range sweep but there is no useful output above a few hundred Hz, so the captured signal just has noise for much of the swept range. Do a sweep to 500Hz or so for the sub and you will see a much lower noise floor.