I think using the extremis would be a great idea and have also thought about this. Im not sure if the G1 can be crossed as low as it would need to be around 1.8-2khz, you might want to consider the fountek neo or even better yet the fountek pro5i which is around the same price as the ac and probably much better. It has higher sensitivity and can be crossed very low. I would also probably go with a large sealed cabinet using those drivers since you have two subwoofers already.