What freq is best for spl? or does it depend on the type of sub? From what i hear, the port should be tuned to about 40hz.

 

Perhaps you've yet to hear this, but there is no precise frequency that works for one and all alike, although you're on the right track with 40 hz, since most SPL-tuned boxes tend to aim 40's and up. There are a great number of factors that affect just what frequency will work best for you, such as quarter wave length, resonant frequency of your car, speaker placement, and a whole bunch of other crazy hoopla. From what I've been told, it looks like your best bet is to find #1 - Somebody who knows their stuff, and #2 - a meter to test with.

Chances are, you'll go through several boxes and ideas, but it's this trial and error that will yield best results.