I have a tempered glass rack that does emphasize the highs and makes everything sound a bit "sterile" and cold.
I solved the problem by having my electronics on some bamboo cutting boards (Ikea Aptitlig) which themselves rest on 4 Moongel pads (the stuff used by drummers to dampen their drums). Works a treat and I don't hear the mushiness normally associated with soft stuff such as sorbothane.