You should never think of mastering as the magical fix it!
“don’t worry it will be fixed when it gets mastered!”
Mastering is the topping on what should already be a peace of well polished music.
There are so many cheap mastering tools on the market these days, that pretty much anyone can master a finished bit of work. The trick to mastering a good album or track is not to necessarily get the maximum output possible, but to simply give it a little polish.
If the mix already sounding good, all you ever need do is level balance the top and bottom and expand it a little to raise the output if needed. The key is, if the track is very open and spacey it would be silly to expand and limit the track so much that it lost its depth and dynamics.
However some tracks like dance music need everything to be as loud as possible so that when they are played in the clubs, they blow the clubbers away with the mass of sound. BUT dance tracks usually have quite a few versions of the final mix (remixes), some cut down for radio mixes and have been mastered so the bottom end has no sub. And other tracks are mastered so they sound good on the home stereo loads of sub loads of top and a little in the middle.
Classical music has a totally different way of being mastered.
You also find that mastering studios (The place where you send your final mix to be mastered professionally) also tend to only do some of the different styles of music, some may only master dance and drum and bass. Others may do rock music and instrumentals and others may only do Classical.
Mastering is an Art in it’s self it’s not just about slapping on a bit of this and a bit of that, as you will end up making the track you spent weeks and months on, sound rubbish in two clicks of the mouse.