ok, let's not confuse louder with "more sound" IE you wont see a decibel difference using the 1, 100 watt speaker, or adding 1, 4x12 cabinet, what you will notice is that you push more air through the 4x12 and it "sounds better" however it still just as loud. Now I have modded a couple mg100dfx (combo's) to accept (wait for it......) 2 4x12, and still use the same internal speaker as well. (that's 9, 12" in speakers kiddies).
now that's the sound, still rattle your foundation at 10.
want louder? get a 2nd set just like I described, now ya got 18 speakers and 200 watts. I wont be telling you how it's done, as I have yet to see anyone catch onto the mg100dfx. but if you're interested, I can do the mod, or take it in and pay a "pro".