I meant to mention this a long time ago and I just remembered it again. When I was trying to mix and master for the first time on my band's album (and I am no mastering engineer for sure) I was using Izotope's "Ozone" for master bus compression and eq. Well one thing Ozone can do is analyze the overall frequency of other tracks and kind of give you a goal to shoot for. Well at the time I used Metallica's "Black" album for the reference. I dont remember which track in particular. Anyway, when I referenced the track, it showed like a straight verticle rolloff at about 18k straight off the chart. How the heck did they do that and is that a standard practice?