Speaker level calibration
Humberto,
First, don't adjust all speaker levels to +10 db. Either leave them at 0 dB or perform a manual or automatic calibration. The speaker level calibration allows sound effect that travel arround the room to maintain the same volume level relative to the listener. Otherwise, it might be loud in front channels, quite in center, loud in surrounds, etc.
If you receiver has an autiocalibration mode, use it.
For manual calibration, most receivers will output a test tone to each channel to allow users to adjust the speaker level. Leave the mains at 0 dB and adjust the other channels so that the volume level is the same from you primary listening position. The best way is to use a Radio Shack type SPL meter, but by ear you will get within a couple dB.
Most receivers also allow you to specify the distance from each speaker to the listening position. Use a tape measure and enter the distances.
Now you are setup properly for the Dolby Digital and DTS soundtracks on your DVDs.