Its OK to not have a load on the amp if it's in standby. Nothing is gonna happen to the amp. Even if you accidently turn the amp all the way on briefly without a load, it's not going to instantly fry your amp. If you are plugging into both your 8 ohm cabs (I'm assuming they are both 8 ohm as is typical with Mesa) at the same time, use the 4 ohm outputs. If using just one cab, use the 8 ohm output.