I have all Classe products, and your connection mistake was, well, not good let's say. Possibly having two volume controls in series for the same amp is the problem.
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The hum on the CP-60 was solved. I asked Classe and they told me to check the grounding on SSP-25 and CP-60. When I checked the power cord, I found out that I use PS Audio Lab II which has its own grounding. I disconnected the grounding of the PS Audio cord, and the hum went away. Moreover, Classe said that my mistake to connect input to input doesn't damage the unit.