Please don't laugh but I was wondering if the Chess board is acting as a damper and the pieces are acting as a diffusor?
I keep thinking the board is acting like a ceiling diffusor just sitting on top of the amp just facing up rather than down.
What happens when you remove the pieces on the board or change the pieces on the board? Was there a change in sound? Do you have different boards that have different weight or materials and is there a difference in sound?
This reminds me of a discussion I had with an older audiophile who told me there was a reviewer who found that their CDs put horizontally vs vertically in their rack and heard a sonic difference in their system. It was some kind of wall diffusor I thought.