Adrian. I am no engineer so my answer is just an educated guess, but I would think its about the total watts available to drive the whole system. Three 75 watt two channel amps would give you 225 watts per channel vs 125 for the first scenario.
Cleaner would depend on the equipment in the signal chain, but most electronics are pretty good in this regard. If you are using pro amps as your outboards then the receiver would be cleaner on paper, but I doubt you or anyone could hear the difference.
I always thought that going to active crossovers is all about controlling the signal going to various channels. I guess in the pro world this would allow you to tune your system to each specific venue.
