Depends if you want something necessarily softer or not,
usually it doesn't concern how low pitched you go; it's more of relaxing your throat, and providing softer consonants.
Also, to get a "dark" sound, try lowering the back of your tounge a little. (Kinda like what the doctor makes you do with the spatula while making you say "aah")
...eh, I hope this makes any kind of sense to you, it's kinda hard for me to explain this... ^^;