glass shelves under components?

I have a Bello stereo/TV stand for my components which are Muse nine sig DVD/CD player, Audio Note Soro SE amp. I have these on brass cones which sit on the glass shelves. The sound had an edge to it until I put some very thin rubber between the point of the cones and the shelf and the sound warmed up to an amazing degree.

My question is: where, on a sound quality list, does glass fall? I presume it is one of the worst materials you could have under your components, and something like wood works better, but this unit worked nicely with my TV etc. Can isolation devices overcome the sound I get to an even greater degree or should I buy a new shelf system?