I think there can be a little too much emphasis on doubling into 4 ohms. This is related to gain and I think the total power of the amplifier is a better way to think about it. There are well respected amplifiers that don't double. Ayre, for example. I'd consider two amplifiers that put out 300 watts/channel into 2 ohms comparable, even if one is rated 75 watts/channel and the other 100 into 8 ohms. The 75 watt one doubles twice while the 100 watt/channel one only doubles once. The 100 watt amp will, however, put out 75, 150 and 300 watts into 8/4/2 ohms.