Hmmm....good question. Here's the spec page, and here are some cabinet plans.........

The grills add about 2" to the depth, and the cabinet necks down slightly between the horn and woofer enclosures, but it is continous internally. One of the real experts will undoubtedly have that number.

I've also been told that if you have the room for a 18-20 cu ft cabinet, you can push 604's down to close to 20 Hz....