Those are excellent links! I was researching this very topic some months ago with respect to soundproofing LFE. That is, how can we exploit the various properties of a wall to maximize attenuation of frequencies below 80Hz or so (and especially around 30-20Hz).
One of the first things I learned is that my simplistic view of the Mass Law (doubling the mass increases transmission loss by 6dB) was only applicable in a specific range of frequencies. In particular, it is the dominant law only in the frequencies between the resonant frequency and the coincidence frequency. This graph shows that:
Essentially, in the Mass controlled area, just adding mass of any type is all you need to do to increase transmission loss (increase reflection) -- the more the better.
Above the coincidence frequency, how stiff the material is starts to matter quite a bit. For the same density, the stiffer the material is, the more of a coincidence dip you're going to see. So you can either make the material so stiff that the coincidence frequency is above human hearing (steel) or you can use materials that are a bit more "floppy" and reduce the dip. Drywall is better than plywood for this region, then. You still need mass (and more is better) but the composition of the mass matters.
A lot of soundproofing efforts, like decoupling and adding Green Glue for damping and the like, are essentially focused on reducing the resonant frequency of the wall assembly and thus increasing the range of the Mass controlled zone.
But eventually you lower than resonant frequency as much as you can and it's going to be in the upper LFE range. Practically speaking, you aren't going to lower it under 30Hz. So now what?
Well, the reading I did indicated that stiffness starts to matter again. I got conflicting reports on this and so I asked about this on Gearslutz, where quite a few world class acoustics experts hang out. I got some fantastic responses.
Full thread is __HERE__
The TL;DRs I got from that thread were like so. First, material composition does matter a lot again in the LFE range, but when they stay "stiffness", they really mean "floppiness" (lack of stiffness). The property that starts really mattering is how "loose" the mass is. That's why if you compare the same amount of mass of sand and concrete, the sand will do notably better at containing LFE.
The other -- and more practical -- lesson I learned was that there really isn't any way to completely contain significant amounts of LFE in a typical stick framed house! The floppy mass that would work so well tends to be either extremely expensive (e.g., lead) or very difficult to work with in any practical measure (e.g., sand). That leaves something like concrete and then you need huge amounts of it.