I understand nothing but groping a bit I found the parade. I thought that the sound could not be like that for the price!
After making the twinning, Mac OS X, by default, provides a format 16bit 8000Hz. The sound is muffled, distant ...
Just go to Utilities, Audio MIDI Setup and select the 2nd Sennheiser MM450-X profile on the left column.
Right click> use this device.
And there it is day and night !!! The sound is crystal! I love these headphones!