Was typing this as hoserama replied..
You'll want to check/setup the current headphone routing matrix, although one should be set by default. The headphone matrix determines what is routed to each headphone channel. Refer to page 108 in the manual.
On the headphone routing matrix screen you can select any combination of each individual recording channel, the main Left/Right bus channels, the Main-out bus channels, and the Sub-out bus channels. Each input to the routing matrix can feed the left headphone output, right headphone output, both, or neither. That determination is made by highlighting the little check boxes on the headphone monitor routing screen.
I think the default setting is just main L > headphone Left out (postfader), and main R > headphone Right out (postfader).. which I think sends all odd channels > left and all even channels > right.. (ch1 > Left, ch2 > Right, etc.). That should work for most things and is always post-fader.
Other than that it can get kind of confusing. Apologies if what follows is too much information. It took me a while to figure out.
You will need to alter the default routing if you: 1) don't have all left channels into odd and right channels into even numbered inputs, 2) want to route a channel to both outputs (effectively panned center), 3) want to eliminate certain channels from headphone monitoring, or 4) want to monitor a Mid/Side pair as decoded Left/Right stereo, which is being or has been recorded as raw Mid and Side channels.
If you want to do any of those things you'll need to deselect Main L/R in the routing matrix and instead route each channel individually, and this is where Pre and Post-fader applies, referring to whether the signal from that channel is tapped before or after the mixer level controls. If set to Pre-fader, there is no level control for that channel via the mixer fader of that channel to the headphone output (its output level determined solely by input trim and headphone volume knob). If set to post-fader the mixer fader does adjust level (in combination with input trim and volume knob). Identical functionality to straight default Left/Right bus routing is achieved by routing channel 1 > post fader left, channel 2> post fader right etc.
So as to not complicate things too much here, more on Mid/Side in another post if you need to do that, which requires pre-fader routing.
Note that you can have multiple headphone routing setups saved and switch between them. This is important to be aware of even if you only use one of them, because if you find something weird going on routing-wise, it might be that a different routing setup was selected by accident. Best to confirm "Setting 1" at the bottom of the headphone routing screen when going in there to check on things.
