While I have not heard of anyone using the m904 for personal (non-studio) headphone listening, I would not think the idea was necessarily absurd -- for example, the m904 has many more inputs (3 rather than one balanced inputs, 2 rather than one AES digital inputs, external clock input, etc.) and a much fuller panel. However, the m902 has some nice options the m904 does not -- a remote control, cross feed circuitry, usb input, a smaller size, the ability to dim the display.
I believe the m902 and m904 have the same DAC unit.
I use the m902 as my primary headphone amplifier, and I still enjoy it tremendously. It is well designed, feature packed, powerful, and sounds great.