Harry, got your introductory pm. Re damping between the boards:
While I agree with John in principle (I am not a fan of Sorbothane), I would, as with all this stuff, let your ears be the final judge. As I see it, because of the way that both boards would be secured tight by bolts to the metal subchassis, I don’t think that rigidity will be an issue. I do think that Sorbo may cause too much damping or you may not like its particular signature. I will include a piece of the material that I used when I made my constrained-layer- damping platforms for my rack. It is a very thin rubberized cloth material that is used in flooring installations that is very effective but not squishy like Sorbo at all. About 1/4 the thickness of the Sorbo sheets that I am familiar with and much more difficult to compress. It is easily reversible if you don’t like it. I find that there are few absolutes when it comes to tweaking, and as much as we would all like to think that our systems are "neutral", most tweaks related to resonance within and outside the system are perceived as positive or negative relative to the particular tonal signatures of our particular pieces of equipment and their cumulative effect. Experimentation with an open mind (ear) is the ticket, imo.
