The one-dimensional concept of "louder is better" has already been mentioned; most mixes nowadays are "pushed" pretty hard (compressionwise) even before the mastering stage – maybe that can explain why the difference is minimal... there's simply not always a lot of headroom for a mastering engineer to work with (although a good mastering engineer should care enough to tell you if this is the case!).
Also, regarding different speakers; remember that different rooms (and speaker placement for that matter) also affect what comes out of those speakers, and in turn what your ears register (muffled/crisp/?..). In my experience there's always a difference when comparing mixes in the studio and in my living room, even with the same speakers.
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