OK, this is my spin on it. Although I don't like to put words in other people's mouths, there is a lot I think Massive Mastering would say if he wasn't basically a nice guy. It's easier for me, because I am *not* a mastering engineer, unlike John. Although there are many steps to a finished professional recording, they are generally broken down into 3 phases- tracking, mixing, and mastering. Each phase depends on the work done in the previous phases.
If you give John, or anyone else, a poor performance, badly tracked, and badly mixed, he will do a ton of work to make it sound a tiny bit better. If you give him a good mix of a well tracked excellent performance, he will do a very small amount of work to make it sound a lot better.
What I read between the lines is a two part answer. The first part, simply put, is- garbage in-garbage out. You can't make a silk purse out of a sow's ear. The second part, simply put, is- you have to walk before you can run. The truth is that it makes no sense to try and learn mastering without a whole lot of experience mixing, and it makes no sense to try and learn mixing without a whole lot of experience tracking. It's like saying, "Houses designed by great architects have this really cool appearance, you know like- finished. I went to Home Depot, and they showed me that this is done by driving nails into boards, and then covering it with paint. I got a nail gun, and it drives nails pretty good, so now I'm ready to start designing houses, right?"
I found the process very interesting in making my first album. I took 2 years learning to track it, to the best of my limited abilities. Then it went to a perfectly good mixing engineer, who did a bunch of stuff that I really liked, and a few things I was not so fond of. Then it went to a badass mastering house, and damned if the mastering engineer didn't use sophisticated EQ to take a pretty good stab at undoing the few things the mixing engineer did that I didn't like. This is what I call "EQ wars". In the end, I think the mixing and mastering engineers had a somewhat different vision of what the album should sound like, and I needed to give *my* input at every step. As the producer, I had my own vision. One track had to go back for re-mastering, because the mastering engineer needed to have my vision explained to him a little better. In the end, there was a tremendous benefit in having new ears hear the music at each step. I believe that the mixing engineer and the mastering engineer should not be the same person. After you've listened to an album for 80 hours or more, it is very hard to hear it with new ears.
In short, if you have a good recording, and you like the mix, send it to a good mastering house, and it is likely to be improved. It doesn't cost that much. It is not meaningful to ask how you can do what a person with many years of recording experience, a ton of gear, and a half-million dollar mastering room would do to your music. And the main reason is not the gear, the room, or even the experience. It is because he is not *you*. Good luck-Richie