I've gone round n round with myself over the 'dark' characterization. I agree that some will use that word upon first hearing Lamm gear and I can understand that as a first reaction. But the more I thought about it the more I've concluded that 'dark' is not entirely apt.
Sometimes when puzzling over word choice I consider the antonym - does the opposite make sense in terms of what I'm trying to convey?
Here's what I wrote of the M1.2 in my 2015
review:
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. Overall frequency balance lacked discontinuity; there was no coming forward or peakiness from the likes of sopranos or trumpets, and no midbass enhancement.