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Would I be better off with two 4x10s.........or one 4x10 and one 1x15 (with horn)?Each cab will be driven by half of my power amp (2x100w tube) and I only need to compete against guitarists who're putting around 20-50w, and I can rely on house PAs to help fill the room if I need more.

I had good results with using 2- 4x10 cabs together. You'll have plenty of tight, focused bottom-end, without getting boomy-sounding.

I guess it depends on what you like. I prefer 15's. If you're running 200w tube, you can go with a 4x10 or a 2x15 or 4x10 1x15. If you're not set in your ways you might want to pay attention to what that 15 can do for you. Don't listen to what people tell you- listen to what the speakers give you.