I'll certainly explain.
This is what I came up with after trying to figure out why an extra .3mW made my Aiwa so much louder than my Panasonic, and louder than 5mW Sonys. Also why the Panasonic was pretty much level with Sony, but with less mW. So I figured that it had to be something do do with the unit's rated amp impedence.
So to me, when Sony claims a 5mW amp at 16 ohms, that means when you plug in some 16 ohm headphones (is there such a thing?) you'll be able to get 5mW per channel running through the 'phones. This also made me think that if you were to use cans with twice the resistance, you'd only get half as much power running through them. But to work out, say cans with 40 ohm impedence I came up with this:
Sony standard headphone amp (not E909 AFAIK) is 5mW x2 @ 16 ohms.
Therefore to find the power supplied to 40 ohm cans:
16/40=0.4
Then multiply the amp's rated output at 16 ohms by the above number, in this case, 0.4:
0.4*5=2mW per channel.
For my Aiwa:
32/40=0.8
0.8*3.8=3.04mW per channel.
For my Panasonic:
22/40=0.55
0.55*3.5=1.925mW per channel.
For the new MJ15:
32/40=0.8
0.8*3=2.4mW per channel
For E909:
16/40=0.4
0.4*3.5=1.4mW per channel
For most Sharp units:
32/40=0.8
0.8*5=4mW per channel.
It works for any impedence, but the lower the impedence of your cans, the more favourable Sony units will be over Panasonics. However, the higher the can's impedence, the more equal they become. That is what I've found. eg, for D66 eggos with 24 ohm impedence:
Standard Sony:
16/24=0.66
0.66*5=3.33mW per channel
Standard Panasonic:
22/24=0.9166
0.9166*3.5=3.208mW per channel
For 16 ohm headphones:
SS:
16/16=1
1*5=5mW per channel
SP:
22/16=1.375
1.375*3.5=4.81mW per channel
Just for fun:
16/300=0.0533
0.0533*5=0.266mW per channel
22/300=0.0733
0.0733*3.5=0.2566mW per channel
This could also explain why people find that cans such as KSC35s sound so soft on their PMDU.
I have no idea if this is right, but when I came up with it, seemed to explain a lot.
P.S. Sorry this is long, I hope it is clear enough though!