If you have a Xterra pro does it take 1/4 inch plug you shouldn't need a adapter if it is like the reg Xterra series, if the headphone you are trying have 1/8 jack then you will need a adapter to go to 1/4 to fit in the jack.
now if it uses the 1/4 you shouldn't need a adapter, but now some headphone are mono and some stereo and some has a switch to change from stereo to mono that can fix that. As you will see that most headphone have this switch.
One of the things I have seen is many time when this problem arrives where they have this switch and will work on one that is a mono or stereo and not on the other is one of the wires in the cable is broke. As you know the bisset problem with headphones is that one of the wires get broke in the cable so Gray Ghost make a headphone called the NDT (No Down Time) that has the cable replaceable in the field so you can keep on detecting if you have a extra cable with you.
The biggest problem is when they make the stereo or mono detector and if your detector is mono and try to stereo headphone and they don't work, there is some headphone company that know make a set that are wired to work on any stereo or mono with no switch.
I never had one of the Xterra pro, but uses and sold may of the reg Xterra and alway made sure they had a set of Headphone that had the switch to change from mono the stereo and a set that didnt need it as they were wired do work either way.
Good Luck and hope this will help
Rick