Ideally, the Sub should be placed out in the open for the best sound. But if you have to put it in the cabinet, your current position is probably the best for the most airflow. Also, be sure to listen for any rattling in the cabinet while the Sub is playing.

Either where you have it or flat/flush against the back of the cabinet. Try both and see what sounds best, trueplay it in both positions to see.

And never close the doors to that cabinet.

As @Airgetlam makes a valid point it probably defeats your purpose which is to hide the sub (for whatever reason). You should seriously think of another way to conceal your sub if that is your goal. Place it behind a chair or sofa or in a corner out of the way. You might even consider placing it flat under a chair or soda. Any of the above IMO is a better option versus inside a cabinet.

Something I did years ago was to pop the center panel out of a door and replace it with speaker cloth so I could hide the speaker behind the door. might be an option the spouse would like better than the open doors.

Another thought on cabinet mods, make a V shaped backer to sit behind the Sub that just touches the cabinet sides near the doors. That would direct the sound more to the outside and possibly act as a horn, reinforcing the bass. 

Thank you everyone for the feedback!