If isolation isn't working, try better bracing.
It sounds that you don't have a strong enough projector mount or poor ceiling bracing. You should be looking for a tight fit onto a well braced ceiling stud to reduce vibrations, not trying to float the pj in midair. Give us a little more description of your bass system. I run 2 ported 12" subs tuned to 17 hz with over 300 watts and never have pj jitter except when someone walked above the pj in my basement install.
Try more bracing, I bet your projector is acting like it is on the end of a whip being popped and I would think that your ceiling is moving less than the pj.
Think of why some people use vibration reducing feet on turntables while others simply use wall mounted shelves. 2 different approaches.
Just my .02,