Mastering is rather complicated process demanding high level of attention and increased aural perception. Human ear has a tendence for temporary wearing out during the day and therefore perception may be changed depending on time and/or overal level of fatigue. Therefore professional mastering should take at least 2 days including first day of no more than one hour of work with the track + the next day when we should listen to the same track with the fresh ear. We cannot also overrate the potential importancy of matching the track we actually work on to the well-mastered source that belongs to the same genre. As far as we can generally learn the sound of our loudspeakers, amplifier and other elements of the audio path and therefore be able to recognize the right proportions in audio signal, the subjective differences coming off the ear fatigue may reach even 4 dB for specific (mostly high) frequencies, which seems to be a very high value, considering precission and fineness needed for making the right mastering job. For a medium-talented mastering engineer, a difference of 0.5 dB at any frequency should be perceptible, while the normal listener is able to notice the difference starting from 2-3 dB.
Above the 4 dB of level difference, listener starts to feel discomfort of perception and that's the thing mastering engineer tries to avoid. Generally, process of signal leveling is called compression and it's achieved by the electronic devices called compressors. The rule of compressors' action is quite simple: the level of loud signals is reduced while the level of quiet signals is raised. Although it sounds quite simple, there are tons of nuances and deatils that doesn't even come to mind for the normal listener, while decent mastering engineer should be able to know them all (well, at least the most of them). There are many available models of compressors and many differences beetween them. Recently the subject of compression is highly exploited following the new standards of mastered track, those demands much more effort and fineness during the mastering process (and last but not least bigger wallet). Few years ago a possesion of one high-end compressor, or even "ordinary" Finalizer from TC Electronic, was quite adequate base for professional mastering work. Today, posession of two high-end compressors is necessary minimum to provide right dynamic enhancement for the modern music.
Another device in mastering, due to some of engineers the most important, is an equalizer. It's raising or lowering the level of the signal for specific frequency of an audio bandwidth. Simply said, with equalizer we can ex. "add some bass". Normal listener certainly should know the equalizers (mostly graphic equalizers), from the stereo car system or simple boom-box. These equalizers are designed for home use and they much differ from the professional equalizers, mostly with quality and the way of working. The most popular in home use are graphic equalizers with predefined bands. For professional use there are mostly parametric equalizers (primarily designed by George Massenburg) where we can define the band and the stroke width (marked with letter Q). Exactly as with the compressor, possession of only one equalizer in mastering studio is insufficient. Additionally, equalizers are more expensive devices than compressors and sometimes the expense of $3.000 is not enough for one unit.
Luckily there are also persons who buy this equipment following the passion they have, not looking for a bunch of clients with large wallets knocking to their doors. In today's reality when the word mastering has became very popular and overused, almost every music producer has an ambition for doing mastering process by himself. There's no place and time for long talking about differences beetween mastering done with software plugins and mastering using equipment worth thousands of dollars, just the one analogy which is coming to my mind is like a compare driving a A-Class Mercedes with driving Maybach. The difference isn't about comparing a quality of single plugin to the simmilar hardware unit, this kind of comparision may give simmilar result. However, today's mastering can be done only using few or even several chained units. Unfortunately when we chain few or several software plugins all we can get is phase distortions, loss of transients and overally squshed sound. In case of hardware devices the situation looks exactly inversely. Every next unit in chain make the sound more full and rich, in the end coming with impressive result, not possible to be achieved in software world. Unfortunately these solution cost - however with a little help from friendly mastering studio, we can get nice results without spending fortune on very expensive equipment.
In sum, we should put a dot over the letter "i" - we cannot overvalue professional mastering. However, we have to remember that even best sound engineer will be not able to fix badly mixed track. If there were used bad quality plugins, microphones, instruments or another mix elements, the mastering engineer will be able to fix that only partially or won't be able to fix it at all. So, as far as during the mastering we are obligated to use the highest quality equipment available on the market, in case of mix (especially in electronic music), we can achieve good results with budget solutions sometimes. Of course we should rememeber that things like microphones or other elements of an acoustic path should be also the best possible quality. During the mix we should remember to leave headroom and dynamics and do not destroy the mixdown with cheap compressors and limiters, leaving this to the mastering studio. We are aware of the fact that many sound engineers dreams about putting a final compressor on master and squashing it with a maximizer, but we should ask ourselves if that's what we care of is a perfect sound, or we just want to play with cheap plugin at the end of work and have a little satisfaction, practically groundless, because it could sound much better... No doubt, we should ask ourselves such a question, at least sometimes...