Your preamplifier is not only providing attenuation, but it also helps match input impedance to properly drive your amplifier- regardless of level.
Not saying a passive won't work, as your Cary amps input impedance is relatively high, but it is not accurate to say your pre is doing nothing but attenuating the source signal.
Not saying a passive won't work, as your Cary amps input impedance is relatively high, but it is not accurate to say your pre is doing nothing but attenuating the source signal.