archon cab watt ohm

I got an Archon 100 but only have a 50w EVH 2x12 thats 16ohms. I remember reading in the manual that you should use a different ohm setting if setting the amp to 50w mode. So should I be setting it at 8ohm or 16ohm? The cab is already 16ohm, and I can only pick 4, 8, 16 ohm modes.