What is the PM16 -- a power amplifier? You speak of turning its volume "all the way up". Does the PM16 have input level controls? If it does, it's commonplace to have those at or close to maximum in order to utilize the gain from the power amplifier.
Normally, the signal level on the Marantz pre-outs is controlled by the volume control of the Marantz. Where are you setting the volume control of the Marantz?
Are you sure you've connected the PM16 to the pre-outs of the Marantz and not mistakenly to the tape outputs, which yield a fixed low-voltage signal that is NOT controlled by the Marantz volume control.
Axiom Resident Expert