The difference between poor coherence and great instrument separation?

Coherence is, how well the mix sits together. For example, recording an orchestra in a concert hall should inherently have very good cohesion because the musicians are all playing together in a single acoustic environment, hopefully well balanced and tuned and of course the instruments in an orchestra have evolved over the centuries to work well together. Many other genres of music have varied sound sources and probably not all recorded at the same time or same acoustic and these sound sources may not be very complimentary. For example, a drum kit is not very complimentary with a singing voice, the frequencies clash and the balance is poor, so we have to boost the volume of the voice do a bit of EQ and maybe some other processing, to make the voice and drumkit sit well together. This is a pro-audio definition of cohesion, not sure if the consumer or audiophile worlds have a different usage.

Instrument separation is, how well and how easily can you hear all the different instruments. In an orchestra, how easily can you tell the violins from the violas or the trombones from the horns? A very badly recorded and mixed orchestral performance might make it almost impossible to be able to tell the difference between say an oboe an a cor anglais, which are harmonically and positionally very closely related. In other genres of music you may have say lead, rhythm and bass guitars, does it just sound like a wall of guitars or can you easily pick out each different guitar? What decides this is the artistic decisions by the producer and the skill to implement those decisions, maybe the producer wanted it to sound like a wall of guitars! So in this example, a playback system which gave great instrument separation between the guitars would actually be a poor system.

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