Chung,
Dan Wright has some mods for the 5900 and is preparing some for the 2900. Even if you're not interested in his mods, he has provided some of his impressions of the audio performance of these units
here. If you go to a subsequent post in the referenced thread, he provides an off-the-top-of-his-head guess comparing the 5900 to the "Sony units".
In any case, don't underestimate the performance of your C555ES on SACD. I have one as well and agree that it outperformed the 2900 that I had in my system for a short time.
By the way, he does good work on his mods; I have a modded 47Ai from him.
Doug