I'd seriously go to a place like Wal-mart, Menards, Lowe's or Home Depot and get some little rubber furniture pads. If you look on their websites, most of them you'll see are a soft, fuzzy material, but they do carry rubber ones in the store. They look similar to the rubber feet you posted in your amazon link. I bought an 8 pack at Wal-mart a few months back for like, $3.50 and they work great. I've even used the fuzzy furniture pads before. There are, easily, cheaper options available other than those rubber feet on amazon.
Check out this product list. Most of the pads have an adhesive side, so what I did was bought a large pack of them from Wal-mart and then would stick two of them back to back, so there wouldn't be any adhesive getting on my speakers. Then, I just put one under each corner of my speaker. Worst case scenario, you don't like how they work, but at least you didn't spend much money.