If your talking about designing these for HT market, then I would suggest you worry a little less about how deep they go, and concentrate on better detail and accuracy, as most people will be working with a sub crossed over @ 80hz give or take...
So if you could get them to roll off around 50hz, or even 40hz you'd be doing great...
BTW, those are very very nice looking cabinets, I would say some of the best looking I have seen...
What is the intended full width and height of them... ?