Quick update,
I just found out that my Grados are 32ohm, with them I get a lower level than with my 120ohm headphones
So:
32 ohm Grado, lower than with 120 ohm Sennh. (loud enough though)
120 ohm Sennh. give me the highest sound level
250 ohm BD lower sound level than with my Sennh.
So this is in line with your findings..
Good point! No idea what the impedance of the outputs is,.. low I guess, maybe not low enough?
This I didn't know, maybe explains it: "
The greater the output impedance the greater the voltage drop with lower impedance loads. This drop can be large to enough to prevent driving low impedance headphones to sufficiently loud levels. A real world example is the Behringer UCA202 with a 50 ohm output impedance. It struggles with some 16 - 32 ohm headphones." taken from:
http://nwavguy.blogspot.nl/2011/02/head ... dance.html