Elevating subwoofer about two feet off the floor

Geoffkait unless your subwoofer weights over a ton the last thing you want to do is "mount it on springs." You will not isolate other equipment from it at all. Get a recording with test tones. Put on the 20 Hz track and turn it up. If your system will produce it at all the entire house will start shaking including the refrigerator and your wife. Low frequency sound waves are very powerful. You ever been around a sonic boom?
Subwoofers are best served locked down tight. There is an issue with the stiffness of the floor. Concrete is always best. 

Uh, there is no 20 Hz information on virtually any recordings. Most people are lucky if the get flat FR down to 35 Hz in the room. Putting the sub on springs greatly reduces energy of all frequencies transmitted from the sub cabinet via the floor to the front end electronics. This is especially true for speakers and subwoofers since the Fr of the iso system is much lower than the lowest frequency the speakers or subs can produce. By the way, as shown in max Townsend’s YouTube video, isolating speakers or subs reduces cabinet resonance as well as isolates front end equipment. Two, two mints in one! 🤗

mijostyn541
 "entire house will start shaking including your wife."
Perhaps now I won't need the movie and wine?