Bookshelf speakers on actual bookshelves?

I have to put my Mani 2's on actual built in bookshelves (sp?) which are also tied to a set of drawers. The wood built in has many hollow spaces and is large enough to wrap around a corner. I am fearful that the Mani2's bass response will either vibrate or deteriorate. Does anyone have experience with what to put under the mani 2-or similar bookshelf-to minimize the problem and maximize the sound? (This speaker is a two way with a port in the back. Electronics will be of high quality.) Thanks in advance.
You might want to try Polycrystal cones or Final Lab Darumas under the speakers for isolation. This will help dampen the vibrations.
I used some poster-type clay to keep them away from the metal shelf and keep them positioned correctly.

You might also try damping the back wall of the bookcase with some towels or sponges temporarily to see if you have a reflection problem with your rear ports.

My little office speakers from a mini-system were very hard sounding and, after experimenting, I went with some self adhesive felt sheets.
poster puddy from any hardware or art supply store...