What's under your bookshelf speakers?

I recently picked up a pair of ProAc Tablette Reference 8 Signatures in bird's eye maple for my office system. I would like to protect the finish on the bottom, but also preserve/enhance the sonics. At the same time, seeing as how this is in my office, I would like to keep things visually discrete. Currently, I have a small mouse pad underneath each one, and in truth, this seems like it might be a viable option. However, I haven't tried them with another product underneath them or with nothing underneath (don't want to damage the finish), so I am not sure what sonic effects the mouse pads are having. The system sounds pretty good as is, though.

Thanks, Tom.