Amp problems? Ohm questions? Amp blowing up for no reason, READ ME!
According to the post above it is possible to use a 4x12 & 2x12 cab together.
I have tried this myself and it delivers the sound/volume you are after. I'm a little unsure of the damage it can do using this over time though.
"At what impedance should I set my amp if I have a 16 ohm cabinet, and want to add an extra cab with 8 or even 4 ohm impedance, running both at once?
Short answer: set your amp to 4 ohm!
For one 16 ohm and one 8 ohm cab, the formula is:
1/ (1/16 + 1/8) = 1/0,1875 = 5,3
So the combined impedance of both cabs is 5,3 ohm. The closest setting on your amp is now 4 ohm.
Again, 4 ohm on your amp is the closest setting. If you do not have a 4 ohm setting on your amp, I would not try this and stick to using just one cabinet. Or maybe two 16 ohm cabs resulting in a combined 8 ohm load."