i can answer this as i have a roland CB100 cube...
firstly the cube is a great lil bass amp with some really great and usable sounds, for 100 watts it is great..
the extension cab feature IMO isn't as great as it seems tho, yes alone you get 100 watts, with the extension speaker plugged in you get LESS! the internal driver is wired in series with the extension speaker jack socket, so when you use the option you effectively run the amp at 16 ohms, thro the internal driver and the extension speaker, probably using 60, maybe 70 watts, its not all bad tho, ive used my cube with my warwick 2x10, that cab has a better low end than the cube's driver, but i would not say that it makes the setup any louder (or significantly quieter), what you lose in watts used you gain in speaker cone area.
i have not done any real testing of Db levels, just speaking from personal experience of what i hear.
i have done a lil surgery on my cube to bypass the internal driver completely and driven my fearful 15+6, sounds fantastic, definately louder and deeper than the cube own driver, not enough watts to get the best out of my fearful tho