or has electronics at this level not evolved that much outside of "subtle refinements"?
In the 1980s there wasn't a single amplifier design made that had sufficient gain bandwidth product to support its feedback across the entire audio band. As a result, as frequency is increased, when designs meet that limit, feedback falls off (on a 6dB or greater slope)- so distortion increases with frequency above that point.
That's one of the reasons solid state has a reputation for harshness and brightness, and a reason tube amps are still produced.
But, solid state has evolved so that a solid state design can be as smooth as the best tube amps thru the mids and highs (where its important) because it is not possible to generate the gain bandwidth product needed so that distortion vs frequency is a flat line across the audio band. This relates directly to smoother sound due to how the ear perceives distortion.
I would not call that incremental or 'subtle'.