Regarding CAST versus OP Amp they are just different not one versus being better than the other. When switching between the two with the same source CAST is a bit more neutral with the sound level slightly lower than with the moon OP Amp. The Phoenix is more refined than the C2. I can't really A-B my C2-SA and Phoenix but going from the Phoenix to the C2-SA I still find the C2-SA pleasing, with detail and a good sound stage. The C2, when I had it, was always pleasing. Yes the Phoenix is better but is it worth 3 times the price better than the C2? That depends on how much your willing to spend and if you plan to ever go balanced. If I where to by a third headphone amp I would have a hard time not getting a C2.1 due to it being improved and compared to the Lyr which I have not yet heard I know I will for sure have a black background with the C2.1-not that the Lyr doesn't I just haven't heard it. Honestly if I bought the Lyr it would be mostly out of the novelty of having a tube amp to experience.