Try figuring out how much power the headphones take for an SPL of 90 dB.
Yamaha should have a sensitivity figure ( how much voltage they require for s specific SPl) or an efficiency figure (how much power they require for a specific SPL) published somewhere.
3 Watts is more like what it takes to burn the headphones down.
I just looked these phones up on the 'net, 94 dB @ 1 mW.
So 1 mW will actually be fairly loud!
the average headphone really doesn't need much power......
Actually, I think its the 10 W Maximum burns-'em-down...
Asgard II on high gain (somewhere around 750 mW into 150 ohms) is loud, but runs out of steam in a lot of passages. Especially base heavy, complex stuff.
One question I have is how is that 94 dB @ 1mW measured? Is that a 1 kHz test signal? Maybe the bass response is much less than 94 dB @ 1 mW?