[font=ttrebuchet ms]As mentioned, heavy strings tuned low require higher action to avoid fret-buzz. About the only thing left to try is shimming the nut to raise it a few thou. This may allow you to lower the bridge a bit more to trade off higher action near the nut for a slightly lower action closer to the bridge. It depends on where you're fretting the strings when they buzz, and which frets they're buzzing against.[/font]