I think I understand what you are saying here however I'm not sure about the chosen adjective.
When I choose adjectives to describe sound characteristics I often consider the opposite of a considered word to see if the word makes sense in the context of the description as to what it is not. The antonym of 'dark' is 'bright' or 'light'.
Here is what I wrote about the
M1.2 in 2015:
"Some find the overall sound of the M1.2 as slightly dark; and by contrast with certain amps there is that relative difference. I found the M1.2s sounding whole within themselves, and from the perspective of their overall presentation, music teemed with harmonic information, with the antonym of their tonality being not light but lean."
Ask different dictionaries about the adjective 'lean' and you'll get different results. In the one I'm looking at now -- thesaurus.com - the strongest antonym of lean is 'whole'. That strikes me as more appropriate than 'fat'.