Competitively priced with the Townsends is the Boenicke Swingbase, which has been around since 2010. 6moons has a preview for a review going on and here's a video of the base in action:
So many ways to skin the cat. No intent to highjack this thread but to point out that if one wants to really isolate something from below, this is one alternative that goes even farther than springs.
All the best,
Nonoise