Dude, you probably know more about BP's than 99.99999999% of people on here.
I find it odd that they're unloading at higher frequecies though. Are they unloading or is the FR just falling on it's face? Your ratio is probably too high and the FR just falls off a cliff after your peak.
I know you know this, but the larger the ratio, the more gain you'll get, but the smaller the bandwidth.
Shrink the sealed section to get resonance up. Shrink your ratio to maybe 3:1 and use 1/3 - 1/2 cone area for the port. Sq in per cube isn't the best way to figure that. That should open up the FR some and still give you a nice gain.