I have 2 12'' MTX thunder4000 subs, each in their own closed box. I was wondering if they would sound better if i port the boxes or if i leave them closed.

 

Ported enclosures are a bit more complicated than that. Ported boxes are usually larger than sealed boxes for the same subwoofer. If your sealed enclosure is the proper size for your sub it will most likely be too small for optimum ported enclosure performance. Sealed boxes are very forgiving in this regard. Good results can still be had if the box is a little too big or too small. The same can't be said for ported enclosures. If the enclosure isn't the right size and if the port isn't the right size you'll get poor sub bass performance.

If you do go ported you may want to look into getting a sub-sonic filter. Subs in ported enclosures tend to flap out of control below the tuned frequency of the enclosure. A sub-sonic filter removes those ultra low frequencies and helps keep distortion down.

 

What may help out your system is if you buy a box that will house both subs. And in doing that, you could buy a ported one. Subs will respond better and will give you a better sound when putting them together, based on experience.

How are your subs set up in your car right now, which ways are they facing?

 

The subs are in the trunk angled up and towards the front of the car.