If I mixed and mastered for your system, you would love the disk but no one
else would buy it. Problem is that the disk will be played on your or my good
stereo system, a good or dismal boombox, a good or midling mp3 player from
downloads or wripping, or worst of all, the radio (but without that one, it
may never be played at all!).
So an engineer must try out the mix on a variety of playback systems and try
to make it sound great on all of them. Possible?
I, for one, use a limiter, usually set soft at 0 and boost the signal till a
few transients go into the red. If it is too quiet, I might boost more, maybe
a little compression, but that's it. Little pretention about airplay since I
really have no "distribution' (A&R as opposed to Linux). I posted a few cuts
around if anyone is interested.
(When I have the $$, I might try to really master and print disks, but again,
minimal intervention. Harbal is a nice tool but not free or GPL, and he may
have a linux version someday as well--the demo works 100% using WINE which is
something in and of itself!)