With used gear, your guess is as good as ours as to the condition. As hd mentioned, it could be the input selector switch. It might be something further downstream.
Double check all connections, just to rule that out.
Have you tried the unbalanced input?
Input sensitivity or overall gain may be less than the Classe. That may be an issue, although the Mac input sensitivity seems pretty standard (2.1v unbalanced, 4.2v balanced). What preamp are you using? If you're feeding the amp a balanced signal, are you using a balanced output from your pre-amp?