Top on Russ,
Those are actually extremely important settings. As well as the test tones. A receiver can be plugged anywhere and when connected to speakers the receiver will make sound. Only when we tell it exactly how far each speaker it to your listening position AND go thought the time to level the test tone volumes (using a decibel meter matching each channel to be the same output volume) have we calibrated the receiver to the environment. Every room is different but if we calibrate the receiver to the room then everything gets better. Slap, attack and sustain. The three qualities I first look for when setting up a 5.1 set up.
If you set it to zero your lower octaves would arrive at your ears before the rest of the sound. This would dramatically effect slap, attack and sustain to the negative. Choose a 5.1 track that you know well and also know it to have good imaging. Say Master and Commander cannon ball scene for example. Play with the settings to validate. If your sub was on a shelf sitting next to your head in your listening position then it may sound perfectly normal. Possibly a bit high on attack and slap but then a zero or a one may be the correct setting. Pay attention to phase as well. If your sub was next to the couch aimed at the front of the room where the television and front array was you would want a 180 phase. If it was sitting up in the front with the front array aimed at your listening position you would want a zero phase.
Hope it helps
Daryl
CEDIA certified