the small rubber bumpers can be purchased at any building supply and will help to de-couple the speaker from the shelf. Always have the speaker slightly forward of the shelf edge so not to obstruct the dispersion. Ideally your channel level should be set to 0db/+1 db, you may need to lower the output of the other speakers to achieve the best results. Also, try a different crossover for your center, try 90Hz or 100Hz, listen if this makes a difference.

Brent Tombari
Axiom Audio Expert