What is the Flat settings on the PowerStation?

The flat setting refers to the shape of the impedance curve as seen by the amp. The toggle switches on the front panel allow this to be modified. Sometimes flat sounds best, sometimes other settings sound best depending on your amplifier, cab, volume level.

Take a look at the full answer here.

It’s the simplest way to describe the fact that those positions are not reactive… high frequencies and low frequencies are not being accentuated by the reactive load because the reactive part of the load is turned off…

Set it where it sounds good to you, set it where it feels good, set it where it sounds right, it doesn’t matter whether it’s flat or not. It only matters what works for you. Does it sound right, does it sound familiar, does it match your expectations? That’s really the goal you’re trying to accomplish.