While you'll get some lower frequency extension with the 6x9s, the oval cone and the coaxial design will cause more distortion at high power in the mid range when compared to a round set of components. The frequency extension is a definite benefit for the 6x9s in a system without a subwoofer, but, with a sub, a component system will have less distortion and will have a flatter frequency response curve - this is where my sound quality comment comes from.
If you're going for spl/volume, I completely agree with you. Since the 6x9s have higher rms wattage and sensitivity, they will be louder.
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