Does one use the egg crate foam, or, if you stuff with polyester pillow stuffing, does one still need to use the eggcrate foam as well? Once I get the amp for this box, I want to stuff it. I already have some eggcrate foam and some pillow stuffing, which would you recommend I use?...
As that's a vented box, I wouldn't stuff it. You can line the walls with the eggrate foam, and that should be sufficient to damp unwawnted mid/high frequencies. In many cases, even that is not required. Stuffing in a subwoofer is done only to increase the apparent volume of the enclosure, such as for a sealed box that's smaller than the size required. In a vented box, you don't want anything that can interfere with vent operation. So even if you were to use pillow stuffing instead of foam, you'd want to staple it to the walls.
You don't really need to worry about overdamping it. Neither eggcrate foam nor polyester stuffing are good at absorbing frequencies below 100Hz. Lining all the walls will only serve to help reduce midrange and high frequencies, not cause you to actually lose bass performance.
All that said, as both your woofer and vents are downfiring, midrange and high output are not that great a concern. Assuming the subwoofer is sitting on thick carpet, the carpet will tend to absorb any mid and high frequences that get by the crossover. And THAT said, my woofer is front firing and my ports are upfiring, I have zero stuffing/lining in my box, and can't hear squat out of the sub besides 80Hz and below, through the ports or driver, using a standard THX 80Hz/24db per octave crossover.
My guess is that you'd have a tough time telling any difference at all between zero foam and foam lining all the internal walls, assuming your crossover is the same as mine. You might be surprised to learn just how many high end commercial vented subwoofers have no lining inside the enclosure.
So darnit, now I am going to have to take out my woofer and throw some lining in the box and see if I can notice any difference. I suppose that unless you get exotic like Black Hole damping pads, adding foam or bonded polyester to the walls falls into the DIY category of 'it can't hurt, and it's cheap, so might as well do it!'.