Re: sub box for a ford ranger ext. cab
With two 6.5" midbass speakers in a 4th-order bandpass enclosure the Fb is much higher than the usual Fb found in a ported enclosure. However 87 is a tad bit high... even for small drivers!
It's been years since I have been really active in car audio forums. I posted a ton about 10 years ago here on caraudio.com, termpro.com, sounddomain.com, caraudioforum.com, and a whole bunch more that I no longer remember. You may remember me. If not, you're probably just young haha. Because of the many years since I've done any selling / buying, the only person I can remember for my references is Team PSI (David Moore).