At what ohm's is best to play my subwoofers and why?

Answer 1

Ohms is the impedance of the woofer. You cannot change that. You can change the load to the amplifier. Two 4 ohm woofers in series would be 8 ohms. Two 4 ohm woofers in parallel would be 2 ohms. The speakers should be hooked up to the amp outputs to match impedance. The amp should either have terminals for different loads or a switch to match impedance.
Hooking up a lower impedance can cause the amp to drive too hard resulting in distortion and likely damage to the amp.