3 cu.ft. tuned to 20 hz is optimum for the O Audio 500 amp. The high pass filter needs to be set at 20 hz which also has about 4 db of boost. A slot port needs to be 2" x 6" x 50.75" long to keep the air speed acceptable. This is assuming the Xmax is 29mm, you should contact Cerwin Vega support to verify this before you start anything. It will take some figuring to get that port in the box.