Best sub isolation?

I have had good luck with damped springs. For damping, encase the spring in thin-wall heat shrink that is shrunk just enough to define the shape of the spring....i.e., not sloppy but also not too tight. By just covering the top and bottom of the spring with the heat shrink that should protect both the bottom of your subwoofer and your floor, or add a furniture foot protector under the spring for additional protection. Some also stuff a bit of foam inside of the spring for damping.

Sizing the springs is a bit of an art with the main parameters being diameter, length, compressed length, maximum load, and spring constant. Here is a source that offers a wide variety of springs, sells in small quantities, and provides a nice tool to help you choose the right sized springs for your situation. Keep in mind that one end of your sub is likely to be heavier than the other, which can be handled by the positioning of your springs, by placing an extra spring or two under the heavy end, or by using springs with two different maximum load capacities. As hints to sizing, try and end up with the compressed length being the same when all the springs are positioned and loaded, and try to run the springs near the middle of their operating range. There is a picture of springs encased with heat shrink damping material on my system page, and also a picture of springs positioned under my main speakers.  I also use them under my two subs.

Springs are the best solution, for sure. The method mitch2 describes will certainly work. It is however a lot of work. I went through it and have some springs. You are welcome to them. For about what it will cost just to ship them you could get a set of Nobsound springs. These work just as well but a lot better because you don’t have to calculate anything just add or subtract springs as needed. The leftover springs can be used under other components. Easily the best value in bargain vibration control.

If you want a lot better Townshend Pods or Brackets are a lot more money but worth it. If what you really want is isolation then Nobsound. If you also want improved sound quality and are willing to pay for it then Townshend.

In between, the other stuff you are considering, costs a lot more but not any better than Nobsound. Only worth paying more if making the jump to Pods.

Good call on the Nobsound springs, definitely easier to implement and more versatile.
Setting up individual springs may take a bit of trial and error.

Townshend, but for that sub, I think Nobsound would do just fine. I would try 4 springs to start with, but since the sub is not that heavy, 3 per foot might be fine. I had to put 3 tiny drops of superglue beneath the rubber pads that go under the feet of my subs to keep the pads from migrating.