An alternative:
Just go into your "setup menu" and select "INPUTS">"SOURCE LEVEL" and change both your "ANALOG INPUTS" and "DIGITAL INPUTS" to +12.
This will allow your volume to read -12 when you are getting the 85dB SPL.
PS: There is no good reason not to just leave the Input levels as they are and just turn the volume up. The control will top out at +18dB, but that is 103dB. If you follow the instructions above, +18dB setting would equate to 115dB.
PS2: My directions above are based on the Denon 4520. Your Marantz is similar, but I'm not certain the menus are labeled exactly the same.