I would think that separates were a contributing factor. At some point, trying to get more and more power, along with tuner and preamp duties, out of a single, massive box, becomes impractical. Most (all?) monster receivers were wider than 19", and heavy. If you really wanted a boatload of power and were willing to spend that kind of money, then at some point separates start to make much more sense. Look at the bigger Sansuis (22000, 33000), they were already getting to the point where they had to break them into pieces to make them manageable.
Along with that, which speakers needed more than 300 watts? Sure, there were a few, but again, at that point you may be starting to look at biamping. For that, separates become a necessity.
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