just exactly how did you hook the head up to the combo to just use the speaker? Does the combo's speaker have a speaker cable that you can hook up to the head? Did you make sure that the impedence's matched between the head and speaker? or, did you run the head's line out into the combo's fx return, or the guitar's input? Please tell me you didn't do this, because if you don't have any speaker's plugged into the speaker out's of the head you could fry the OT (output transformer)! and this is a very bad thing!

I suggest you get a 2x12 cabinet with two 8ohm speaker's and wire them parralell so the final impedence is 4ohms and make sure you set your head's output for 4ohms. Or, you could wire them in series so the final impedance would be 16ohms, again setting the heads output to 16ohms.

Whatever you do, make sure that the impedance of your head matches the impedance of the speaker cabinet you're plugging into.