.... that the anechoic response of your sealed sub should be somewhere between the red and pink curves.
In your sub's case, more stuffing will result in an over-damped response while no stuffing at all will result in undamped internal standing waves. The subwoofer should be fine as is if you're already satisfied with the sound. For Qtc values of 0.707, or maximally-flat response, the effective acoustic volume of the box should be around 54L (or less for a frequency response with a bump or peak).