That certainly seems true, the FAD's are very source-dependant, probably because they are so transparent & revealing. That description of the 1350M2 matches mine - heavy bass and rolled-off treble. Could also be that different tips have an unusually large effect on the sound with the FADs. Also the fit in the ears is critical - on the first ocassion that I listened to them, I thought the 1601 had a peak in the lower treble but that turned out to be because I was not wearing them correctly. In short, because the 1601's are so transparent & revealing, everything becomes more critical - source, ear-tips, proper fit in the ears etc