I use 3 pucks under my preamp as well. The pucks I'm using are good for 20 pounds each. See link below. My guess is the preamp weighs around 35 lbs. FWIW, they do say to have the logo pointing toward the front (in other words, toward you). Not sure if it makes a difference or not, but that is how I've always positioned them.
http://www.isoacoustics.com/iso-puck/
http://www.isoacoustics.com/iso-puck/