Standing waves resulting from room modes can have the effect of making it unbearably loud in the spot where you're standing, yet mere feet away, causing a null where the bass disappears. They may also make one note boom out much louder than others. Where you place your amp or cabinet is critically important in this regard. Real world solutions on typical small stages include using two cabinets (or one combo amp and one cabinet) placed at different distances from side walls, corners and the floor to minimize such effects, and lower the odds that you'll inadvertently place your amp in the worst place in the room.
So there's a legitimate basis in physics for buying that extra cab you're GASsing over!
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