i have mine pointed up, probably with the diffusers on the second or third "click" from flat. they are at about a 45 degree angle.
if you are having trouble with them, try moving the nozzles up, or rediecting the flow so that they converge above rocks (not directly above sand). that way when the two flows meet, they will form turbulence that will be disrupted by the rocks before it reaches the sand. you will probably have to do some experimenting to find the right combination of direction and angle the results ins good turbulent flow without disrupting your sand bed.
that said, i have to do some work on one of my powerheads to keep it from digging up a corner
but that one discharges directly against the glass
oh, and i have 5 powerheads in my tank. my two MJ1200s are on all the time and my 3 MJ400's are on a wave maker. the 1200s are both pointed in the same direction, and the 400s are pointed in the opposite directions... so they converge all over the tank...