Nidri, I contacted the company, they no longer import Auralex stuff, but I read and those make a whole lote of sense acoustic-wise. I have voiced my deslike of 'magic' or unexplainable science, which tends to be very strong in the audiophile community. I'm not judging that approach, I just prefer not to follow that path, but rather things I can understand with what I know or can investigate of physics and electronics.
This pads clearly isolate the speaker by providing solutions for two concepts: providing a dense solid support which can cope with the energy moving the speaker (if the speaker moves a lot, it will 'lose' energy wasted in moving the cabinet, it needs to be more or less still), and provide a dense foam that will absorb contact vibrations and prevent them passing through to the shelf.
Also, what I mistook for Cork originally is apparently MDF. I could replace my DIY isolators with that further down the road.
For now, I must report success with my current project:
I tried different combinations. The thing is, I can 'measure' the isolation by putting my ear flat against the drawer's side (solid wood) and covering my other ear.
The findings where, as you guys expected and suggested, remarkable. When no isolation is used, I can hear a lot of bummed bass, as well as 'muddled' middle and high frequencies. That is very indistinct 'boom, boom, boom', mixed with hssshhh and mmmmmms on the high and middle frequencies.
With the isolation, two very interesting things happen: the 'boom, boom, boom' is still there, but much less so, and also much more defined, not so bummed and indistinct (cutting very long low frequencies is no easy thing even in large recording studios!), but the high and middle frequencies disappear altogether from the drawer.
That tells me one thing for certain: the drawer is not getting those frequencies any more. What it doesn't tell me is if the setup if now getting those frequencies clearer onto the room, or if the mat / cork are eating up those frequencies. Logic, size and my ears, tell me that they are doing what they are supposed to. I don't hear less frequencies.
I do sense a more defined bass which is most welcome, and I think there is also less 'contamination' on the middle frequencies. High frequencies are as defined as always.
I really think this is the right direction. Perhaps refining the depth and density of the materials will prove better or worse, but the idea works and I am really happy about the more detailed bass I was so clearly missing during my first post.
Of course, there are also more hours of break in into the system, so perhaps that is helping as well.
At any rate, there is a measurable influence of these changes, so I am confident they are helping in some way.
Thanks for pointing out that as a probable source of problems in my setup! Much appreciated!