hang-the-9 :
Depends on the speakers being used. Only way to know is to measure it with a meter.
No, not true.
Every speaker has a SPL which can generate an amount of db at 1w. My speakers has 101 db spl so every time you double the power you add 3db like this:
101db at 1w
104db at 2w
107db at 4w
110db at 8w
113db at 16w
116db at 32w
Ofcourse that high frequency speakers has a higher spl then mid or bass speakers. Usually a 25cm (10 inch) low frequency speaker has around 80-87 db SPL when a tweeter has 110-120db spl.