Harmonic balancing corrects the frequency imbalances found after the mixing process, thus, it can produce more impact while retaining a smoother, more pleasant sound. This also helps prevent listeners from experiencing the effects of 'ear fatigue' due to sustained hours of listening. The concept is especially true, when it's also desired to obtain a maximum  delivery potential {MDP}, and if necessary, reach ridiculous loudness levels by means of hyper-compression.

When a song is "harmonically balanced", the results range from subtle to dramatic, producing an optimum, seamless translation on all playback systems. The goal of this process is to 'enhance' frequency bands, or correct existing frequencies that are in conflict with the natural sound spectrum, and to improve upon the separation between instrument sounds {and/or vocal parts} found in any recording. In our opinion, it is a the most important step in the pre-mastering process, and more accurate results are best achieved with the guidance of spectrum analysis. 

Note: we are one of the few mastering services (perhaps the only one) that apply harmonic balancing strictly by spectrum analysis, unlike the majority of mastering professionals who do this by ear. There is nothing wrong with that approach, in fact, it is the oldest methodology, but arguably, it is the least 'accurate'  when you consider that a single high peak, fast rising transient event, sometimes it can't be heard, but can spoil what could have been a perfect record

What is the difference between harmonic balance and harmonic enhancement?

Harmonic balancing deals with the frequency distribution across the spectrum to obtain the most perfect translation on all playback systems. The task of making a record sound loud and smooth on every system is daunting. For example, a record may sound terrific on a particular set of speakers, but terrible on another. The main goal is to get to an optimum point where the balance between the two results in best translation even though the physical aspects of the delivering medium are completely different.

Harmonic enhancement is the addition of harmonic 'distortion' to either a group of frequency bands {be this sub sonic, bass, mid-bass, middle range, mid-hi or top end frequencies} or, the entire frequency range. The result may also be 'harmonically balanced' but that's not an absolute and your miles may vary.

Also, harmonic enhancement should not be confused with 'sonic stamp'  which is the introduction of harmonic distortion inherent in all analog tape machines, signal processors, vacuum tube systems to a sinusoidal alternating current, and which results with the addition of noise from circuitry oscillation; an effect some people regard  as being 'analog warm'.

In conclusion: when a mastered mix has a great harmonic balance, the sound translates better on any playback system. There are several case studies that have proven without a doubt, that prolonged periods of listening to bad , even mediocre mastered recordings, lead to hearing fatigue which takes away the fun and enjoyment out of the music. Users of our mastering service report that our process results in a smoother product; one that encourages anyone to listen more.