The Gramma is larger and costs about the same as the Subdude. But I believe they don't use the same configuration on the bottom for the foam. You may wish to ask them what the differences are.
I am using two (2) Subdudes under each of my VTF-3HO's. I took off the rubber feet to allow the subs to sit squarely on top of the Subdudes. This should't be any problem with the STF-2 since it is smaller. The spiked feet on the STF-2 will sit on both accoustic treatments with no problem. Measure the distance from each skiked foot first to make sure it will fit on top properly.
You will notice a significant difference with and without the treatment. They will "decouple" the subwoofer from the floor and help prevent some of the annoying rattles in your home.