Usually, you are questing for the flattest response, perhaps gently sloping up as the Frequency Drops (such as the popular Harman curve).
Consider that with speakers, you are usually looking for no more than a ±3dB variation across the range of the speaker. If you are able to achieve that In-Room, then that is excellent. I would argue that the same rule could/should be applied as much as possible to your Sub response.
That may be optimistic, but as a goal, it seems worthy.
Your response above, the purple looks the best, but at 60, everything starts falling apart.
What did you do in the Purple Measurement, and what changed from the Green/Blue measurements?
