I did that with the AVT100 combo and it rocked! The cable youl see on the back that goes from the speaker to the amp can be pulled out. You then get another cable and hok the cab up to the input where the loudspeaker was plugged. It means your no longer using the combos speaker but instead the cab.
The results... AMAZING!! the cab with the AVT100 I had sounded better than the cab with my current JCM2000 head! Go for it!