It is all imprecise, or "blurred," as you say.
Much of it is prejudice and nonsense. Imagine two power amps. One is rated as putting out 100 watts continuous RMS into 8 ohms, and 200 watts continuous RMS into 4 ohms, with a thick aluminum case. So we have a nice amplifier rated as doubling its power into 4 ohms, in a nice solid, heavy case. Imagine another power amp, rated as putting out 150 watts continuous RMS into 8 ohms, and 200 watts continuous RMS into 4 ohms, with a steel sheet metal case. It should be obvious which one is "high end" and which isn't. Yet they could be identical performers, as ratings are claims, not measurements, and when the first is measured by some revıewer, it may exceed its ratings into 8 ohms, which will further enhance its mystique. Indeed, they could be identical amplifiers, just in different cases, marketed to different people.
In practice, you will find that different people will make different claims about what is "high end," "mid-fi," and "low end." Most* of the distinctions are probably prejudice and nonsense.
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*This note is added for the morons who imagine that "most" means "all" and excludes any exceptions. These people should learn to read, but experience has told me that many morons post online and respond to posts as if they said something they very distinctly did not say and so here is this footnote. I won't bother responding to the idiotic responses that demonstrate a lack of understanding of the difference between "most" and "all."
