Judging by pictures of the subwoofer that I found, you might be able to gain entry by removing the grille cloth and exposing the driver. From there, it could be possible to unscrew the speaker and tilt it out of the cabinet or remove it entirely.
While this may not let you get at the board, it may let you see how the cabinet is assembled. I would not be surprised to see the components placed inside and the cabinet sealed with glue afterwards. You might have to break the glue to loosen a panel and get inside.
William