I'm surprised how often I see these secondhand
The sonic quality of the Carbon is subtle. It does not "come at you" and "grab your headphones" and ears the way many other amplifiers do. It takes a little longer to appreciate, unlike some of the more popular brands of headphone amplifiers. And it may not do particularly well at meet conditions. It is definitely suited best to many hours of enjoyable listen while at home, or due to its small size while on travel.
I also find that it works quite well with the highest tier of headphones. Think along the lines of HE-1000, HD800S, etc. I suspect many people may be looking at this as an affordable Cavalli amplifier, which it is! However it still truly shines best with the best of the headphones. And the best of the sources...those who are stretching their budget to buy this may not have either good transducers or good sources.
Thus I suspect these qualities are what makes the turnaround so high. Consumers are not using this like the high quality amplifier it truly is. Give it some time, and it'll grow on you.