I took another look at the manual (on the McIntosh Website) and the part I quoted is from the remote control description. Elsewhere it says the the front panel mute mutes the loudspeaker outputs. So apparently the mute on the remote control and the mute on the front panel functions differently.
If you are not getting the volume you want, could it be that the McIntosh doesn't like the impedance of the headphones? High-impedance headphones may not sound as loud?
If you are not getting the volume you want, could it be that the McIntosh doesn't like the impedance of the headphones? High-impedance headphones may not sound as loud?