There is usually a little bit of play in the way the empeg sled is mounted to the dash; and the dash will flex a little bit under impact as well, giving you yet another layer of protection for the player.

Back in the days of in-dash CD players without electronic shock protection (remember those?), I found that this didn't work the way you'd expect it to.

If you had a CD player that wasn't solidly mounted to the dash, in other words, if it had a bit of "play", this would actually increase the chance that the player would skip. In order to help keep the player from skipping, you needed to mount it as solidly as possible to the dash, with absolutely no "play".

This allowed the player's internal shock absorbing mechanism to work properly, without the additional secondary sub-vibrations caused by the player's casing bouncing and rebounding a second time within its frame. Also, the vibrations produced by a loose stereo mount are at a much higher frequency than those created by the car's motions.

I would assume that the empeg is in a similar boat: You should mount it as solidly as possible in order for its shock absorbing tray to work properly.