General rule of thumb is at least as far away as the diameter of the port. Since you have a sub you might try them plugged vs not and see if you have a preference.
Your center isn't in a very good place and should at least be pulled up even to the edges of the cabinet, and perhaps the tweeter aimed better at your listening position (can't see if it's angled in the photo). I'd move the L/R speakers the same, up to the edge of the cabinet (and same, are the tweeters aimed at your ears?).