I feel the same way but theres lots of post that say this amp gets louder using all the inputs but I tend to agree with youAs long as you have enough signal going into a single pair of inputs to drive the amp to full power I don't see how using both pairs of inputs could make the amp any "louder" .
As long as your subwoofer output on your radio is non-fading and high enough voltage to drive the amp to full power you would see no benefit from using all four inputs. The manual eludes to that a bit.I feel the same way but theres lots of post that say this amp gets louder using all the inputs but I tend to agree with you