Selecting a tuning freq for SPL competition is heavily influenced by the unique cabin gain produced by your automobile. The best tuning freq for SPL comp probably won't give you good SQ. You could sacrafice some SPL and tune lower for better SQ. But that's not practical if you want to succeed in SPL comps. Port plugs are a way to get two different tuning freq. from the same box. One for SPL and another for SQ.
Originally Posted by trujap
I've seen ports placed in many places and have never really heard any big sonic advantage specificly due to port location.
As far as one port for two subs in one camber, you don't have to place it in the middle. You could place it on the right or left side.
For a 4" diameter port try to keep it around 4" from any surface inside the box. This helps with air flow and turbulance. Although if the port is slightly closer you probably won't notice a difference.
Pioneer 8600 - Profile Amp-Infinity Speakers - L7 Subwoofer