I wouldn't worry, yes the desk will vibrate, but glass is more resonant than wood, it will react (ie vibrate) much more anyway. but it won't shatter, it's actually more likely that intense high frequencies will do the damage as opposed to lower frequencies of bass. I sometimes help set up sound rigs for events, concerts, club nights etc and have often had speaker stacks running into the kilowatts (RMS) sitting right by windows running all night.