I have exactly the same speaker setup and I was very surprised at how strong the 500 is. Mine is set at around 5:30. One tool that I found very helpful was the Room EQ Wizard. Here is a link to an article of someone using to help determine subwoofer placement.

For me I had to turn my 500 90 degrees and switch the phase to 180. Could not believe the difference that made. You will also be able to see quite clearly where your trim should be set. Other than that, your spl meter is your best friend.