Just tried the 6XX’s from my Magni 3 from a Modi 3 and I could not listen to it turned all the way up as it was starting to be painful. Though not the most accurate way of testing I have a calibrated mic hooked up to a old iPhone and with a little Blues Travellers I got 111 db with the mic parallel with the pads and my hand over top like I said probably not real accurate but I doubt too far off.
If you are listening that loud that will become a problem for your hearing.
The problem could be the Go Xlr not putting out enough voltage for the line out this will definitely not play as loud as it could. Did the same test with a Dragonfly Red and it gave me 99 db and it was louder than I tend to listen too but it was not painful and it puts out 2.1v which is spot on for standard line out and using it as a dac for the Magni was close to the same as the Modi as a dac.