I was working from the gain instructions provided at this link from Crown:
<http://www.crownaudio.com/media/pdf/133472.pdf>
It's interesting to note that the "volume" knob on amplifiers do not all function the same way. It appears that Crown and Europower amplifiers adjust the input sensitivity. If Crown is to be believed, in this scenario it's better to have the gain turned up on the front end leaving the amplifier with as much headroom, while minimizing noise, as possible. The results in the system at my church were almost unbelievable when we made this change. We also rewired the speakers and I think that was significant factor as well.
There is no level adjustment you can make on an amplifier that will give it more "headroom". That is SOLEY limited by the output stage.
The best you can hope for is to LOWER the input noise from previous stages-giving you more "dynamic range" by making the noise floor lower. But it does nothing to get any louder.
It is very rare that turning up the gain or sensitivity of an amp will improve the noise floor of the system. Usually turning it down and driving the previous stages harder is what give the best noise performance.
I highly doubt any speaker rewiring made a "significant" change. Unless there was something REALLY SERIOUSLY wrong in the first place. But even then-I find it hard to believe it was "significant".
At least that is my experience. Others may vary.