I'm more used to working with home speakers where you often run the woofer to 1-2KHz. However I've also had the understanding cone breakup happens at all frequencies. I know 10" woofers of all materials including aluminum have a large amount of breakup even below 1Khz and to me, thats mid-bass not midrange.
tho as you said, 80Hz is too close to being directional. Thats why I typically cross subs at 40-50Hz. unfortunately its very hard to find drivers that will produce clean loud sound below 60Hz in a car. So i typically aim for 60-70Hz crossover.
