Current efficiency of a SPL car

[d3cibel]

Do we need minimum current draw at the tuning frequency ot the maximum current drawn by the amplifier at tuning frequency ?Measured by a DC clampmeter

[amc32]

The lowest impedance to the woofer out of the cabniet should indicate fs.
Fb is determined by the ports which you are using. But does not indicate directly the lowest impeadance acheivable by the speaker.
Run a impeadance plot of the enclosure with woofer and/or woofer installed.
Find the impedance at Fb this will give you a indication of the sort of current draw you should have. But don't confuse this maximum power output of the amp. Since this is a SPL vehicle your quest is dependant on a limited band-with.
Do the plot,find the freq.,find the ohm,do the math.
Hope this helps