This perforated, rubber drawer line is very good for sound and vibration dampening. There are several manufacturers of it. Available also in other colors such as taupe.
Free air, tighter bass - snake oil or cheap tweaks?
Usually when we talk about snake oil it's because some one is out to make a buck on the gullible. In the case of these tweaks I want to recommend I'm not going to make any money, and you may not spend any either, but I find them useful.
Cover your Speakers
I don't know why a modest change in your acoustics can make such a difference but I usually find that covering my speakers adds a lot of air and room ambiance. I like to use some thick curtains that I've had from a house a long time ago. Works really well.
Weigh Your Speakers
I don't mean put them on a scale, but put a weight on top. Of course, this doesn't apply to those with behemoths. I find this especially useful with lightweight speakers that are on stands or very small footprints. Think 2-ways, both bookshelf and floor standers. I think the additional weight resists the tendency of the woofer to move the box back and forth, creating a kind of Doppler distortion in the bass. In some cases I find this tweak can really make a speaker sound tighter and clearer from the mid-bass downwards.
Clean up the Floor
We often pay attention to the walls, but not the floor, especially behind speakers. I find that room treatment, cushions and blankets here can really cut out hash I didn't know I had.