How loud should my subwoofer be?

[Romulus]

Alright, I just received my Harmon Kardon speakers this week and finally set up everything. I popped in Kingdom of Heaven and flipped to the siege scene and left my subwoofer fairly low. I didn't really hear anything coming out of the subwoofer so I then turn it from approx 1/4 to 3/4. I replayed the scene and literally the entire floor was shaking. I thought that was good enough so I just left on that. Then when I played one the battle scenes from The Patriot, whenever a gun would fire I could feel a thump in the floor. It seemed abnormal to me so I turned it back down again afraid that I am going to ruin my subwoofer. So my question is how loud should I have my subwoofer? Obviously you can't tell me exactly how far to the right the knob should be turned, so if you could tell me when I should hear the subwoofer kick in, that would be great. I'm just afraid if I have it too high all the time I might ruin too soon.

If you need to know any specifications for some reason:
Receiver: Onkyo 605
Speakers: Harmon Kardon HKTS-18 5.1Speaker System

Also, if you have any other tips for "Do's and Dont's", it would be greatly appreciated.