hi Andyr
Probably the silver sheet works as a shield against RFI unless "pyramid powers" do their work here
I hope the tubes can still loose their warmth enough.
My GK-1 is not build yet but I was thinking of bringing the tubes inside the wooden box, which is easier to shield. Then I have a cooling issue as well. So I will make little holes for air ventilation thru the shield. A Faraday Cage may have little spaces inbetween its nodes.
Try making pyramids of metal(silver) sheets with some little punctures to air the heat out and let us know. Since you are walking the path of the absolute I would certainly use Herbies tube dampers. They will make a big difference even in power amps. I will order them certainly, as soon as I start building