well most people will tune a port at a low frequency .. around 30 hertz.. really deep bass .. the reason is that once something plays below this frequency the driver cannot control it which is why a subsonic filter is used to block the frequencies.. however everything above the tuning frequency will be louder than say a sealed box .. how much louder depends on a lot of factors of course but usually around 3 db.. a longer port will tune the box at a lower frequency.. length depends on size of the box the driver and size of the port..
The driver is a fancy way of saying your subwoofer or speaker, yes a subsonic filter will block out the freq. below what its set at. man i saw a site like that the other nite let me look for it again if i do or dont find it i will let you know. i would just go sealed and say the hell with it. i like sq over spl anyway