It's all to do with where the manufacturer sets its reference level. Domestic music systems tend to use the No Sound point as zero (0). As you turn the volume up the sound gets louder and the numbers on the dial read higher and higher. 10 is often the maximum, but if you've ever watched Spinal Tap then you know that 11 is '1 better' ;-)
In the professional world or recording studios, film production and movie theaters they have a different way of measuring sound. They have a "reference level". This is point where the sound is at its optimum volume for dynamics and best quality. What recording engineers and projectionists are interested in is how much below or above reference level the sound is. Reference level is 0. Quieter than reference is the negative numbers. Louder than reference is the positive numbers.
This idea of reference level measurement has been adopted by the home theater market. It makes sense in some respects because of how the speaker set up procedure works. One speaker is used as a reference and the others are matched in volume to it. Many kits use the front left speaker as the reference level, then the remaining speaker volumes are trimmed +dB or -dB to make the same volume of sound.
When setting up your LG kit the +6dB is like 11 on the volume scale. It is absolutely at the maximum of its adjustment range relative to the other speakers. If your other speakers are all in the +dB area too then you need to go back and do the sound set up again. Settings + or - a couple of dB is more usual relative to the reference speaker.
Bass is hard to set correctly first time. Most systems I get called to go fix have the bass set way too high. It thumps bangs and generally smothers everything else. Try a lower setting on the bass speaker and move it up against a solid wall but keep it away from the room corner. Bass should contribute but not dominate.
Once the sound set up is done correctly then the master volume control takes over. This will adjust all of the speakers up or down in volume and keep them in balance.