Subwoofer with little Xmax, vented or closed box?

Bass Habit Elite E300D1, enclosed or vented box? Ive heard that subwoofers with bigger "xmax" will move more air because they can flex further, however the E300D1 only got like 13mm of "xmax" which, i assume isnt much? I compared it to another subwoofer that had waaay more xmax but less rms, which one would move more air?

I got 2x of these E300D1 in a pretty good vented box, and they do move SOME air, but id like more. So im wondering if another box could give better results or is it only new subwoofers that will make a difference? One thing is that these take 1k rms EACH, but the amp i got (avatar 1000.1) can only do 600w in 2ohm, so they are not getting used fully right now.

Had a friend claim that my amp could easily go down to 0.5ohm even if it says 1ohm on the box. I assume that if you have good and stable power to the amp it could work, but i only got 1 battery, and it at most drops to 13,5v when playing full volume, but do jump around 14-14.8v when there isnt much bass. Risk it at 0.5ohm or just use it at 600w?