I know this is an old question but answering it since i ran into the same problem.
It seems that the TRS outputs on the studiolive rack series are impedance matched, which means that, unlike a normal balanced output where one ear of the headphones will be in-phase and the other in 180° rotation, the outputs on the 16/24/32r will only display in phase audio on one ear.
To solve this you can open up the PM1 and solder the wire from pin 2 on pin 3 and that will give you mono on both ears.
Now there is probably a discussion if you should run headphones on a line out without amp, but that's another story @presonus probably knows if this is a big no-no on these devices? I seem to get pretty decent output on a pair of 250ohm headphones plugged in directly (L+R bridged to the tip of the trs)