If the shelf is large enough for Auralex MoPads or something similar you can, but I'd still want them strapped on somehow in addition. I would look at some of those brackets that have "clamp" style mounting where they cradle the bottom of the speaker and have rubber to keep the speaker from slipping out. Do they happen to have a mounting insert in them on the back? If so, Omnimount has stainless ones that can hold 10, 20 and 30lbs.