Ya, I've brought up that concern with many speaker engineers including, Scott(Fi), Nick Morgan, David Moore, Nick W.(IA) and others who have ran/seen similar setups.
They all had said that as long as the woofers are all doing the same thing at the same time(in phase), there will be no cancelation at all. I was also concerned with as much pressure there will be excerted in that area if the cone bodys will sustain the mid 20hz region and I was told the Havocs have a pretty superior cone structure and should stand up to it without having to worry about blowing dustcaps off.
I've gathered mostly my info on this build from Mr. Nick Morgan as he had a similar setup in the Nissan, Chad Hix old truck.//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif