The position of your right hand over the strings can make a big difference in the kind of strength and volume you're able to pick the strings with. Also, when you get your thumb out in front of your fingers, closer to the fingerboard, it makes it easier to do palm muting. Palm muting means resting the thumb side of your palm on the bass strings, right where the strings meet the bridge. It gives your bass notes a dry, thumpy sound while leaving the high strings free to ring out.