I thought amps doubled the output at 4ohm, yet there are some rated 200W @ 8ohm and 300W @ 4ohm. Why is that? If there is no 4ohm spec from the manufacturer then how does one determine the output at 4ohms?

How can the A1400-8 output the same (all channels driven) 200W at all impedances?

Is there a way or formula to determine the one,two, or three channel driven output based soley on the all channels driven rating? For example, if a 5 channel amp is rated 200W all channels driven then what would be a reasonable estimate for 1,2,and 3 channels driven?