For some time now I've suspected that there is some sort of antagonistic relationship between timbre and brightness and, because the timbre issue is one of the most important issues to me and brightness is my least acceptable, I seem always to be walking a tightrope between the two.
This is a common problem! The thing to understand here is that the brightness caused by distortion does not contribute to timbre at all- instead simply makes the presentation more irritating, as the ear finds the higher ordered harmonics to be less pleasant.
To get timbre right, you have to have low distortion and pretty good bandwidth, and the distortion has to utterly lack any of the higher orders (5th harmonic and beyond).
In many amplifiers the feedback is poorly applied, the result being that those harmonics are injected into the resulting output. This has been known for a very long time as Norman Crowhurst was writing about this problem over 50 years ago. This is not to say that feedback in an amplifier won't work, but until the advent of simulation, its been almost impossible to do the design correctly using the traditional formulae.
So as a result many designers got it wrong- and the amps they made tend to sound bright. I find that many audiophiles tend to prefer sins of omission rather than commission...