i want to get a kenwood excelon 12 inch single sub. what i want to know is this sub says it 50-600 rms range at 4 ohms and my amp can be set on mono i think like at 350 rms but it says at 2 ohms. will i notice a difference if the amp is 2ohms and the sub is 4ohms or will it affect the sub or amp in a bad way?

You will probably get half the power (175 rms) to a 4 ohm load with your amp set to mono. It will not hurt the amp, but you will not maximize the full power it can provide.

If the sub can handle 600 watts RMS, I wouldnt underpower the sub that much. If would get a sub that closer matches up to your amp, or have the amp overpower it by a lil bit.

You should be ok, just don't play it too loud, you'll clip the amp.