Are they the Bowers 683 S2? Measured
here by Stereophile, shows a sensitivity of about 88dB, which is fine, but the impedance is rather low-ish and with significant phase angles, meaning they're a tough load on an amplifier.
Is the Marantz the HD-Amp1 like
this one at Crutchfield?
At what volume/loudness do you start hearing distortion? Is the distortion in the bass, mids, or treble? How big is your room and how far away are you sitting?