Mass and vibration cancelling on top of CD player?

It all depends on the player. If its something mass market, try putting a big book (textbook) on top and some cones underneath. It can make a big difference.

Why does it make a difference and what should be expected?

For my Oppo BDP-83 SE which I felt vibrated too much when playing I took some extra sound damping material from a PC build put that and a 5 lb weight on top of the Oppo. I also put some computer hand rest wristpads on the feet of the Oppo. It stopped a lot of the vibrations but for sonic performance improvement I'm not really sure because I implemented a lot of tweaks at one time.

I use 5 VPI bricks on my transport and 2 on my DAC.