Yeah it is isn't it - you'd logically expect a measurement directly from the headphone socket to be better.

However, the N900 has separate volume modes for headPHONE and headSET, and my suspicion is still with the firmware: It appears that somewhere along the line gain is being hiked up too much, introducing distortion into the mix - and perhaps it's being done less in headSET mode.

Would be good if someone could find a way to solve it via alsamixer...?