A couple of other decoupling options for subwoofers and speakers include (at a relatively low price point) platinum silicone hemispheres (link 2" and link 2.5"), and (at a higher price point) Equipment Vibration Protectors (EVPs) by AV RoomService, Ltd. (link).
I have used the larger (2-inch and 2-1/2-inch) platinum silicone hemispheres under my subs, speakers, and steel equipment rack for a couple of years now with good results, and I also use the smaller sizes under components and peripherals such as power supplies and digital switches. As an elastomeric material, their response is similar to that of a damped spring, assuming they are utilized within the prescribed weight limits.
If you are good at DIY, the relatively expensive EVPs appear to be primarily made from Owens Corning 703 and 705 Rigid Fiberglass Board Acoustic Thermal Insulation (link). This material can be purchased from a variety of sources and is also sold by Acoustimac (link), a company specializing in acoustic room treatment. The EVP folks do dress it up nicely and sell it as constrained layer footers with a metal and then either rubber or felt facing but, at a much lower cost, you might conceivably try simply setting your sub on a slab of the material. I have recently been doing something similar with heavy components using 1-inch thick dense wool felt.
Here (link) is an interesting series of posts about the isolation topic.