I've personally had cancellation issues with similar firing arrangements. IT will very between vehicles though. Ran 2 similar setups, one it a T-bird, and one in a Regal, the T-bird had ZERO bass, the Regal slammed. You get my point. BUT, alas. . .
Maybe this just isn't the driver for you. Personally, my Mag in 1 cubic foot with 250 watts was enough to make my ears hurt. You may be in need of more. Multiple woofers and/or more power, or a totally different driver may be your problem. I hate to say it, because I really love mine and couldn't ask for more, but hey, it's just me.
I would try a different enclosure though before doing anything else.