My experience was that, when set to -1, ,every enemy in the whole place was +1. All the minions were yellow.
Then I set it to +0 and sanity returned. All the minions were white.
I generally play on -1/+4 for the first 20 levels, with bosses and AV's in single player. The idea is then I don't have to bother with enhancements until I'm ready to place my sets and stuff.
The bosses were wiping me out in 2 to 4 hits.