Spiff (and others),

Those are definitely placed there to reduce vibrations, as you suspected. I don't think they're intended to improve the sound of your amp however. What they do is prevent the lid from ringing and buzzing if it picks up vibrations caused by the music you're listening to.

The stainless steel sink in my kitchen has similar deadening devices glued to each corner on the underside. Imagine the sound of a spoon being dropped into an un-deadened metal sink. Similarly, imagine how cheap the amp would feel if you rapped on its lid and it had responded with a tinny echo.