But that will have to wait just a bit until I have some free time. For right now, I would like to make a couple of comments from an angle that I don't recall having seen mentioned anywhere.
In brief, the story goes that after the high noon of the Classical Style, the next phase of harmony consisted of extending the resources somewhat (though harmonic extremes were certainly known previously) by using more remote keys and modulations. The use of the submediant, the flat submediant and even the minor flat submediant became a hallmark of the Romantic mood. Root movement by thirds instead of fourths or fifths became common and alongside all this there was a general trend toward what we might call "harmonic saturation". Instead of the clarity of the Classical style, where the tonality was always quite clear, except for moments of dramatic effect, the Romantics sought to create harmonic ambiguity by larding on more and more chromaticism. But the notion of gravitation toward a tonic was still essential, particularly in the music of Wagner who, while he stretched delayed resolution to the maximum, still depended on tonality for all his effects. Delaying resolution creates no tension unless there is a real underlying harmonic structure.
But towards the end of the century the weight of the continued chromaticism started to weaken the whole system of tonal relations and with the work of Schoenberg and his students the idea of the "emancipation of the dissonance" took hold. The idea was finally to erase the idea of a tonal center and simply to allow the use of all the notes of the chromatic scale freely. They soon decided to organize a system around a 12-note series, but the idea of free dissonance remained.
After several decades of dissonance, that may have caused a precipitous drop in audiences for the new music, the idea of consonance returned and it is safe to say that most composers today write music that is much more consonant than was common between the 1920s and the 1970s.
The idea of using modal harmony instead of tonal harmony or dissonant harmony is the way that recent music is often described. But, except for isolated cases, I don't think that is really what is going on. I think that what a lot of people are doing is what you might call "diatonic harmony" instead of "chromatic harmony". Sure, it might look like a mode from time to time, but I don't think that many people are really thinking in terms of the traditional modes, what used to be called the "church" modes.
What I often find myself doing is simply writing diatonic music. It is not exactly tonal in the historic sense, but neither is it modal in the historic sense. Why don't we just call it diatonic as I don't use a lot of sharps or flats? I think that Philip Glass might be doing something similar.
In any case, let's listen to some Philip Glass for a sample: