Impedance and Bi-Amping

I have 8-ohm speakers that I want to bi-amp, and both amps have impedance selectors on them. I want to maximize the power the amps deliver, but am unsure what the speakers' minimum impedance is and at what frequency. Any suggestions as to whether I should set the amps to 4 or 8 ohms? Thanks for any advice.
Dear Bojack: The best way to know the impedance curve of your speakers is to ask to the manufacturers.

Regards and enjoy the music.
I'd leave them at 8 barring any additional info.