I'm experiencing the same problem. I used to have an Audiolab pre and amp using a Classe .3 cdp. This had a very warm a smooth sound.
I first switched out the amp with a Belles 150A Hot Rod. Everything still sounded warm and smooth, but more details. The characterics of the Belles that I've read is that is transparent, warm, smooth, and "tube like". I was satisified with this sound, but noticed that the Audiolab was somewhat coloring the sound.
What I did next was replace the Audiolab pre with a Placette passive. Music became more transparent, detailed, and dynamic. Half way through a cd I can notice some listening fatigue or having a "glare".
Not sure if the Placette is revealing the weak link in the system or the synergy is not good because both the Classe and Belles are described as having the same sonic signature (warm, smooth). I know the Placette doesn't add or subtract from my system so I know it can't be that.
I'm not convinced it could be my room causing the brightness, but it might. I'm just trying to avoid having to add room treatment since I know the wife won't go for it.
Since I'm experiencing somewhat the same problem as Mdp0430, my hunch it could be the amp. I've heard the Cary and I can't see that being the issue since I would describe that to be warm and smooth sounding. Very "tube like". Then again, it's probably just not a good match for the Plinius kind of like how my Classe is not that good of a match for my Belles.