Adding decibels requires a knowledge of logarithms. Adding Watts is simpler in theory, or at least it is when you are talking about direct current power consumption.
It makes no sense at all when it comes to the sound power output of Combo amps, where the stated wattage is largely a product of the marketing department.
Make your drummer use brushes or hotrods, or a djembe, a set of congas, or a shaky egg!
Cheers
Dave