There are definitely wall mounts that will work with 8 pound speakers, but your shelf will work and look good too. Maybe you could include a full-width block underneath where you could screw “up” into the back, and down through the shelf into the block.

Consider a small lip around the shelf too, or another method to keep the speakers from falling off. Eight pound speakers could really hurt if they fell on somebody’s head.

I don’t usually worry about stuff like this, but years ago we were on vacation visiting relatives, and one one of their surround speakers rattled off and landed square on one of our kid’s head, resulting in a trip to the ER and many stitches.