sub box advice needed.

Hi all, I'm attempting to build my first Sub box for my Girlfriend for her birthday along with a new Sub and amp. I have the following sub and amp in mind. Amp Sub Now on to my question(s)... I've read up so much over the last week and I'm not really any clearer in my head. What exactly is it that determines what the optimum frequency the box should be tuned to? The Sub I have listed above recommend that the box should be tuned to 26HZ, yet I read of people who seem to choose what frequency they tune their own boxes to rather than what a specific manufacturer recommends so this confuses me even more. We both like to listen to Drum n Bass music with very low Bass tones so should I stick to the manufacturers recommendations or would a different frequency suit us better? What makes me even more confused is the specs of my current sub and enclosure set up. My sub box According to the manufacturer the box is tuned to 50hz, yet after using your calculator it appears that its actually tuned to roughly 33hz. The port is 4" diameter and 11" long. Any help would be greatly appreciated as her birthday is looming ever closer and I feel like I'm not making any progress with the knowledge of what to build.

I would stick to the JL specs. They pay there engineers plenty of money to come up with what is going to work best for each line of there subs. Being this is your first enclosure build it makes it even better to be able to follow there plans. Also the enclosure you linked is a prefab. Prefabs are usually a generalized airspace and tuned for high output over quality of sound. Think of a prefab enclosure as going cloths shopping. Telling them you are say 25 years old and they hand you a size and style based on you age rather than tailoring something sized and to your specific style for you. That is kinda the difference. Different subwoofers use different size enclosure and tune to get the most out of them and for your tastes.