I've been present at a number of pro mastering sessions for major label commercial release projects, and each time always noticed the mastering engineer applying a Low pass filter around 20Khz. Is this a case of getting rid of frequencies you can't hear that are using up headroom, so you remove it to gain more overall volume on single mixes for radio?. Is there a specific reason for LP filtering a track for Radio?. If so, at what frequency do you Mastering Gods concider the upper limit of usefull Khz regarding Broadcast/Domestic systems and what db/octave would you apply the filter at this frequency. Many Thanks Bearclaw.