Mixing cabs is a bad plan. The reason it is a bad plan is that the sound of the combination is totally unpredictable, so as soon as you plan on mixing the cab with unspecific different cabs, you can't get any useful suggestions, because nobody can know how they will pair up, the sound isn't a combination of the souds of the two, its another sound again. Do you like the sound of the 10s you have but need louder? Get more the same. If you don't like the sound of them, get rid and get somehting you do like the sound of, then add more the same until loud enough.