If you've got an eq, try setting the tuning frequency to 25hz. You'll be able to trim up your port which will help you with port noise. Then you can eq the rest out. But, as my *somewhat* golden rule, I don't like to eq more than 6db. If that's needed, something else can likely be fixed with the install. There's no science behind that... just my own rule of thumb. Also, you need to consider the type of EQ you have, and it's capabilities (parametric/graphic, overlapping bands, Q, etc). Sometimes you don't have enough to fix a problem.