I have a few sets of speakers with stuffed rear ports. The bass does decrease, but it also eliminates boominess, so even though you have less lower frequency response, it's often more defined and enjoyable. My guess is that your 8" woofers with stuffed port will at least equal the bass of the Audioengine 5" and the Yamaha 4", and without a doubt, the Audioengine 2.75".
Give it a try. I usually use a strip of 3/8" - 3/4" foam rolled up, but most anything would work, including "stuffing a sock in it". Some people use a bundle of soda straws (which has the effect of reducing the port diameter) and try to tune the port with the length of the straws , but in your case, stuffing the port to change the alignment to mostly sealed is probably what you want.
Steve