Just a sub-note to the power to loudness; that's all other things being equal and the speakers not limiting, cables being transparent, etc.
Not saying it's wrong per-se, but just beware of things which don't state the assumptions being made.
There's another saying that 10dB is twice as loud.
But I've also seen 10x Watts = twice as loud (ie. 1000W = twice as loud as 100W); that's why a 50W valve amp can sound surprisingly loud!