A powered sub's speaker level input "reads" the signal in the same way that a voltmeter reads the voltage in a line (high input impedance). The voltage is not driving anything, but the sub's internal amp tracks the changes in voltage that it sees at the speaker inputs. So there should be absolutely no alteration to the signal. In theory, of course. This is why I originally suggested this route, rather than splitting the line-level input signal from the CCA.