Back in the late 70's and early 80's when there was a large segment of Audio that actually loved the science end, the use of the "decibel watt" ... or dBW ... was a reference to get people to understand what happens as we add more "watts" to the equation.

It worked like this ... 1 watt = 0 dBW. Doubling the power adds 3 decibels, and ten times the power adds ten decibels.

For most of the music range - 100 to say, 5000 Hz, ten decibels "sounds twice as loud".

In other words, if we have a reference of 90 decibels, 100 decibels will sound twice as loud, but require ten times the output to sound this twice as loud.

So the scale looks like this:

0dBW = 1 watt
3 dBW = 2 watts
6 dBW = 4 watts
10 dBW = 10 watts
13 dBW = 20 watts
16 dBW = 40 watts
20 dBW = 100 watts
23 dBW = 200 watts
26 dBW = 400 watts
30 dBW = 1000 watts

The text below has the caveat that a speaker rated at, say, 87 dB for 1 watt at one meter in sensitivity will not be ruler flat at that spec (think +/- 3 dB), and also each speaker will start to compress at a certain SPL ... but it's still a good method for understanding the relationship from "watts" to "loudness in SPL"

This made it fairly easy to take a speaker's sensitivity and the appropriate dBW and come up with total output.

All ratings for SPL will be using the "dB per watt at one meter".

An 87 dB speaker and a 200 watt amp would mean adding 23 dB to the 87 dB, and that speaker would deliver 110 dB at one meter with the 200 watt / 23 dBW amp.

A 104 dB speaker and a 10 watt amp would mean adding 10 dBW, with the speaker delivering 114 dB.

This is why we often see Klipschorns using small tube amps.