You did not say what the problem with soft surrounds is? I cannot see any problems with that, not even in horns.
The Goerlich cones are very light and as I said, have an aluminium foil glued on top of the cone, basically to make it airtight. Kea Audio cones do not need that because the material forms a thin layer on the outside which makes it airtight. Inside, as mentioned it is foam, exactly as the Goerlich/Podzsus drivers.
As I said, there is no advantage of sandwich, it is only used to produce a hollow inside, which in our case is achieve with a foam. YOu can call this sandwich, but because the outer layers have no supporting function and are very thin anyway I would not consider that sandwich (it is a matter of opinion however).
Hope that clarifies it.
I have never heard of any woofer that manages 50Hz lower cut off frequency in a horn arrangement. The simple fact is that a woofer does not emit sound under the resonance frequency (that is without artificially changing that, ie active). The resonance frequency can be calculated simply with the mass of the cone assembly and the suspension (that is spider including surround and air load). So if you make the suspension rather stiff (that also happens if the surround is very stiff) then the resonance frequency goes up. In order to compensate for that you would have to make the cone heavier (that is why the big woofers have a harder suspension, because they are so heavy). The disadvantage of making the cone heavier is that the efficiency goes way down. This is partially compensated by the larger effective cone area of large speakers, but again there is a limit to everything. Putting a compression chamber behind the woofer certainly makes the suspension extremely stiff and you would have to use very very heavy cones to achieve 50Hz. So what is the idea????