The Atma-Sphere M-60 always struck me as pleasingly warm in tone, but not syrupy. This is a wonky comparison, though, because I have no idea if the Atma-Sphere GAN amps convey the same house sound. Maybe someone who has heard both will respond.
I own both sets of amplifiers. I have owned the M-60s for over a decade and purchased the Class D around the time they came out. The Class D gives up very little to nothing compared to the M-60s, simplifies ones set up quite a bit, and if you care about saving the environment and some cash on your utility bill there's that too.
From the average audiophiles perspective the description of the M-60s by @tvad might sound nice, although I wouldn't use the term "pleasingly warm in tone" myself. From my perspective I would say both the M-60s and Class D have low coloration characteristics due to their low distortion, especially in regard to higher order harmonics which tend create unnaturally bright tones. They are both well balanced amplifiers (with AES48 compliant balanced differential circuits) that will pass the "truth" to your speakers. You will most likely find your speakers and components upstream from the amps introducing more coloration. The amplifiers themselves not so much.
As a long time Atma-Sphere customer and authorized dealer feel free to take my comments with a grain of salt.
