To me, for the home recordist, mastering is taking a collection of tunes (wav files) laying them end-to-end in a new recording project and then applying compression and multi-band EQ to them to get a uniform volume between tracks and having a more unified sound. I am on the side where you don't really master a single song -- you master a collection.

Since you don't have a special room setup just for mastering, all you are doing is continue to "mix" individual tunes. If you can't get a good mix that translates to different sound systems, you will never be able to improve that by "mastering" them individually in your same room.

You could send out individual songs to a mastering service, but if your mix doesn't translate very well, then you have limited how much the mastering engineer can do for you. I am pretty sure mastering engineers are just going to use multi-band compression and EQ on your mix, but they are also in a well treated room with excellent monitors and that will help.

Now at bandcamp: Crows Say Vee-Eh @ bandcamp or soundcloud: Kevin @ soundcloud