Hi Giovanni
I too play bass, and I don’t need a separate headphone amp, but if you feel you need more volume than you can get out of the UR22c you can buy almost any headphone amp out there and connect it to the ur22c line outputs.
But maybe your ur22c is not configured properly? Use input 2, set to Hi-Z if you have a passive bass, set gain as high as possible without the red clip light ever comes on and turn the Mix knob fully ccw for direct sound only, fully ccw to use amp simulator/DAW monitoring, or somewhere in the middle to playback from DAW but monitor bass direct.
your probably trying to use too low impedance headphones. the headphone output is very lacking on this interface (its the only major drawback for me). I have status audio cb-1s that sound great and then i have 250 ohm dt 990 PROs that i can barely hear. I have to use them outside of this particular interface. my old lamda lexicon has no problem but its an issue for the ur22c from everything i can tell online.