How many DB's roughly does adding a Ported box gain ?
I have heard and read about both sides of Sealed and Ported Boxes.

As of now, I have done my whole complete first system, but only thing lagging is my box. I picked up a sealed box, which is aprox 2.5.
I just got new subs, and I am almost ready to hear them for the first time. I had to make 3/4'' spacers because my new subs were a bit deeper then my previous ones.
New subs are Lanzar Heritage HRWDC12.

I have a fairly large truck, seeing as it is a 1977 Monte Carlo. Next project I think will be to dinamat s.p? the whole trunk.

 

I've seen some big number estimates like 3dB gains and up from switching sealed to ported... It's hard to say exactly how much you'll gain but a well-designed ported box will definitely show improvements. The amount you gain will be contingent on what frequency you tune the port to, port area, box size, etc. etc.