I believe they don't stack.
Falling down/going down a trapdoor makes you fight harder monsters.
There are events which can make the min/max levels of these trapdoor monsters even harder.
But its kind of like fighting monsters about 5 levels higher than normal.
The main benefit of this is being able to drop higher level drops.
I.e. you can drop 55 items at level 50 without getting lucky on eps.
+1-5 levels per trapdoor (modified by events) and they do stack.
GL!