As you gain more experience listening to music and movies, your tuning-by-ear abilities will improve. You should try to attend some live music events like a symphony orchestra to get a feeling of how music sounds. You will notice that most of the time, the low frequencies enhance the overall sound, but they should almost never be overwhelming or overpowering. Of course there are exceptions. And movies are a different beast altogether.
Tuning by ear will take some time (a couple of hours) and many listening tests (this is the fun part anyways). I like to do a quick listen with some reference material with the sub turned all the way down to see what the main speakers sound like for music or movies. Then I crank the sub toward max and listen to the reference material. You will probably notice that at both extremes it doesn't quite sound natural. With the sub turned down, it will sound weak and incomplete, and with the sub turned up high, you will notice that it seems overwhelming and you can even point out where the sub is located. You will also want to turn the crossover to a low frequency (I think many people suggest 50Hz or lower). This will greatly depend on the other 5 speakers and their frequency response. If they can only go down to 80Hz, then you will need to set your sub a little higher, otherwise, you will have a gap in the sound. You also don't want to crank the crossover up to 120Hz - 200Hz because you will get a lot of annoying bass, especially during typical male speaking. I would experiment with this just so you get a feel of what I mean.
In the end, I like to set my sub to a level that it just blends with the rest of the speakers, so it is not too loud and not too soft. Like they mentioned above, the sub should be transparent.
One reason the bass knob has so much more range is because every room and setup is different. If you are in a smaller room, then you might not need as much bass as someone in a really large room. Also, some people are just bassheads and like to crank it all the time.
If you wanna hear some deep male voices, check out Tim Storms. He has the Guiness record for lowest male voice. He can hit as low as 8hz. Here's a video of him singing: http://www.mitchfewell.com/Amazinggracevid.html
Good luck with your level setting.