Here's the deal. Since you are trying to tune lower, putting bracing in will raise the tuning, but will increase your cone control. Unfortunately, with that power, it won't control it enough to overcome the unloading with the higher response, so you'll need to reduce the port area.
Since you ARE running that much power, however, you won't have to follow any port size requirements. I say drop the air space as stated, close up the port a bit with either a divider or an extra port wall shooting for around 8in2 per cube, and you're golden.