port tuning for my Icon

[Notorious97200 339]

As you can see in my sig, I am a pround owner of a 15" Icon sub!!! I am really happy with it, but I would like to play with the tuning frequencies of my box!

I want to lower the tuning frequency of my enclosure. What should I do ?

The box is about 4ft3 and the port is 14.10" (35cm) long and about 5.5" (14cm) of diameter.

The actual port is just a PVC pipe.

Should I replace it with a longer port(1" more), or is it better to have one port with a larger diameter to have more port area ?

It is easier to have a longer port, but is it better for the sound, output, quality,... ???

Thanks you all