Let me follow this up as I understand it:
There are 3 separate things often referred to as 'dampening'
There is the actual dampening of the case panels to deaden their vibrations. You do this if your case panels are audibly vibrating.
There is mass loading to increase the amount of mass the sound energy must penetrate. You do this to try and keep noise inside the box.
There is diffusion to break up the moving air of a sound wave. You do this to soften how sound waves hit the case panels.
For dampening, I like butyl. You need not cover every inch of a case panel to deaden it - just a few strips here and there - much like a drummer might use a strip of tape or 2 to control the vibration of his drum head. Of course if you cover the entire case panel with it, you've also mass loaded the panel, so you're 2 for 2.
For mass loading, self stick vinyl floor tiles are effective and cheap. I don't think they are quite the equal of butyl for dampening, but they take you a good way down that path as well.
For diffusion, I agree again. Open celled foam is the easiest solution. I usually save the 'egg-crate' bumpy stuff that some computer parts ship with for just such use.
Of course, you could gild the lily and apply all 3, and only add 15 lbs or so to your box
By far the easiest thing to do is to start with quiet components. This will usually negate the need for most case dampening.
this is simplified possibly to the point of being inaccurate, but:
sorbothane for vibration isolation and probably vibration dampening
vinyl for vibration dampening via mass-loading and sound wave reflection
dense open cell foam for sound wave absorption
Another question, what screw size does s-flex "F" use? This will help me in choosing either "fan screws"
Sorry if this has been posted before. Did some checking but no concrete answer. Also, hello to all!