THis has to do with reference level. -15dB is short for 15dB below reference level. If a system is calibrated to reference level (average 85dB, max 105dB, max 115dB LFE), and you are watching at -15dB at your seating position, you will be listening at the exact same level as someone else that is...
Setting a speaker to "small" will redirect the bass below the "small" crossover (varies between receivers) to the sub. Note the crossover is not a brick wall, it is a slope, so the bass below the x-over will gradually taper off (as the frequency gets lower) on the speaker and gradually be picked...