I've seen a couple guys use a ceiling mount that was essentially a pipe flange screwed into the ceiling, then you cut a piece of pipe to the length that you need, spraypaint it black, and use a standard screw mount with a clamp to the pipe, and voila, custom hanging rear speaker ceiling mount that is the right height for your space. This would possibly allow you to put them at the correct distance and height.
Also, if you bought those mounts on Amazon you can return them really easily and pick up these mounts that will not harm your speakers and will screw directly into the holes on the back. I've used these before and they work very well.
http://www.amazon.com/VideoSecu-Univ...+speaker+mount
You can also buy the Omnimounts directly from ascend.