You have it exactly right pitviper. Bringing the drivers a little closer will improve things. I feel like the current configuration is closer to -6db at 20 degrees, but ya, around there. The nulls are expected, because the woofers cancel each other out once out of phase. The way to fix this is to get them closer, or lower the cross over. I could have lowered the cross over, but as we know, everything is a compromise, and this is one I made.
It is limited to the XO region. Above 1200hz you can look to my other polar to see the kind of polar response you can expect.
The reason you don't see this kind of plot with a typical MTM center is because it would look really bad.