Interesting thread in that you first asked the question almost three years ago and based on your post today it seems you maybe tried some stuff and came to your own conclusions since first posting.
Related to your comment about "cushions under the feet of your subwoofers...and speakers...to avoid vibrating things in your room indirectly through the floor", some here believe springs and/or elastomeric materials do a good job of that. I have had good luck with springs, Herbie’s products, and with these elastomer footers (for a lot less money than the other options) that can be sized for anything from large/heavy speakers or subwoofers to electronics and peripherals.