OK. Bob O., you define frequency response and frequency range as helping to distinguish categories. I would add headroom and peak SPL capability to help distinguish. In other words, a “mastering” loudspeaker/amplifier system should have significant headroom so that it doesn’t squash the peaks. One time I did a mix at a studio bringing a pair of decent “audiophile quality” speakers and amp to the studio. But when I got back to the mastering studio I discovered the material needed a bit of compression — the speakers I used for monitoring during the mixing session were self-compressing.
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Which brings me to the question of overlap in your definitions: Can a mastering loudspeaker be used for mixing? You bring up the concept of a “reference” loudspeaker. In your concept, is that a loudspeaker to be used to check balance to ensure that instruments which are at acceptable level when mixing on high quality loudspeakers are at acceptable level/balance when checking on the “reference” loudspeaker? Just to check, but not to mix to the reference speaker, as you point out.
Personally I’ve not run into that situation: if I want an instrument to be subtle in a mix that means by definition it will be even more subtle and maybe even disappear if listening on inferior systems. My client of course will listen and if they have issues with something being too soft or subtle they’ll tell me.