What I would do right now, now that it’s out of the ground. Hook it up and light something on fire that will create a lot of smoke. Push some bass audio in the sub and take the smoke around all edges and such to see if there is a leak.

After looking at a couple of pictures. It looks like the cabinet that is seen from the outside is also the cabinet the sounds reverts off of. So I can only imagine all the bass sound creating some leakage at the edges.

Find them and seal them up.

KOT