There are many things to consider when looking at amp specs. The most important in my mind is the fact that a speaker is not a constant resistive load. The M80 only drops to 4ohm in certain frequency ranges Impedance graph
Amps are not asked to produce continuous high power levels, it is transient and most power is required by the woofers so if your using a sub and not powering the lower frequencies the required power is much lower. This is why sub amps are such high power .
Read the posts on page 4 and 5 of this thread by Andrew, it explains something that is really quite complicated in an easy to understand fashion that even I can wrap my mind around.
I am not sure what other questions you had other than why some amps double their power into 4 ohms. The AdA amps do up to point where the power supply can no longer maintain current to the rails. 3 channels seems to be the limit ,If they used an even larger power supply and 20A circuit they could maintain rail voltage over more channels.
The rail voltage remains constant until we reach the maximum continuous current capability of the power supply, at which time the rail voltage will reduce or "sag". This is why the power output drops at a certain point when more channels are added. At that point the maximum rail voltage is no longer the limiting factor in output power, it's now the supply current and the subsequent drop in rail voltage. If we had an unlimited supply current available, the power output would keep doubling at each halving of the load impedance until we reach the current limits of the amplifier output stage, which has current limiting protection. On the A1500 this will not take place until you're down into sub-2ohm territory.
DOG is GOD spelled backwards.
I blame my terseness on my keyboard.