THOSE DARNED dB's

Can anyone explain the dB (decibel) to me?

I've read page after page of definitions and descriptions. I understand it's a ratio of two values of power, and therefore has no unit (alright with the jokes). I also understand it's logarithmic, that dBSPL is used to measure the "loudness" of a sound, and that dBU is most commonly used by audio engineers.

What I can't seem to find is any consensus on change in the amount of dB's vs. loudness. Some texts claim that while 3dB is a doubling of power, 10dB is a doubling of loundess. Others say 6dB is twice as loud.

The problem is that while a dB is an absolute measurement of power, loundess is entirely subjective and depends on things like the frequency you're looking at and the sensitivity of various listeners. We essentially have to ask people to tell us when the sound seems twice as loud and then look at the dB's. But even if it's subjective, we should still have an average number of dB's that represent "twice as loud".

So what is it? Is it 6 or 10? Or something else?