the Fs is the free air frequency responce. if you wanted to go ported, you can't have a box tunes lower then the driver's Fs. this will greatly affect the SQ of the subs and determine how low you can actually go, in frequency that is.
the actual frequency responce will vary between vehicles and boxes. and the "rated" frequency responce is pretty useless for subs. they are all the same and you only run frequencies between 20 and 80 or 100Hz anyway.
how can you find out what your box is tuned to cause i bought one from a audio shop that they said it would work good with cvrs. can you just measure the port or somethin or how the h*ll do you find out?
yea find the port width by height or if its round internal diameter then go n try to find a site and just start entering in frequency numbers to find out about how close ur box is tuned to