I am in the planning stages of my next system for my car. So far the plan is 2 Adire Tempests in a 8.4cu.ft. box with 2 4"x8" ports. I listen mostly to rap but want decent sound quality without the exspense of Spl(more in favor of spl). The amp will be either a JBL bp1200.1 or a Hiphonics BX1500D. Do these box measurements seem correct for the sound i want? Any input will be appreciated. thanks