Agreed with Luke. I would check with Axiom to be sure however as Mark suggested.
The speakers slide down to lock into place, so I don't think a bit off of vertical will be a problem, but definitely make sure they look fully down into place.
Mine are installed on a standard vertical wall, but because I have young kids and the W150 is easily at their grabbing height, I made sure that it could not easily be pulled off of the wall. They really do lock onto the wall very solidly and it takes a pretty forceful upward motion to free them if they're locked into place properly.
And most importantly - they sound GREAT!
(but they do need a sub)
