Newbie here, been lurking around for a while.  I'm trying to get my first rig working with 2 RX-580 8GB (Red Devil), planning to mine cryptonight. I'm running Ubuntu 16.04, installed the "AMDGPU-PRO Beta Mining Driver 17.40", XMR-STAK and with the RX-580 out-of the box got only ~650 H/s from ea. What should I do next to optimize?

I've seen a a lot of post about custom BIOS, overclocking, undervoltage. But everything I see relies on using Windows tools. Any specific suggestions that I can do in my linux environment?