I'm pretty sure that the harder a level is, the better the stuff found inside will be. For instance, the first 5 levels of the normal mode will only unveil weapons for level 1 to 10 (approximately), while the first 5 levels of the insane mode will unveil level 50 weapons.
But does anyone know how random the stuff is dispatched through the levels ? Is it possible that a specific level 50 weapon can be found in different levels ? Or is it carried FOR SURE by one of the monsters of a specific level (and in this case, if you really want that weapon, you'll have to re-play the level until you get lucky and drop the weapon) ?
The reason why I'm asking that is that I've almost finished the game but there are still plenty of holes in my invertory because I've never found some of the equipment pieces and weapons.