I'm playing with WinISD, and I have a question. What is a good -3db cutoff point? What is the lowest frequency music goes? Also, what is a good top frequency for a sub? I'm trying to get this box PERFECT!
lowest you should worry about is ~20Hz good crossover point for subs is 50-80dB depending on the sub and it's size. Tuning ports will depend on the Fs of the driver, and your goals.
Human hearing cuts off right around 20hz, and not many mainstream songs will go that low. Where the -3db should be is dependant on the slope of the enclosure you are using. Sealed boxes are generally about 12db/octave whereas ported is around 20-24db/octave. If you keep the F3 around the 30hz mark for ported and around 40hz for sealed, it generally gives good results. As for top frequency, the stength and cutoff of your midbass will determine it.
But be reminded, all of this is subjective and you may not like the sound. You need to experiment and see what you think sounds best.