I have several units and it always does my job. The back cover of the battery is indeed not very well made, but it can be easily managed. It has more than enough power to drive your earphones to level that you need. I have used it for many live concerts, relatively quiet and very very loud and never had any problems. The knobs feel okay. It's better to use it with it's adaptor. I always use a new battery in every live performance if I don't use an adaptor. Any wired amp will not be very comfortable. In this respect p1 is as good as it gets.
Finally, always, absolutely every single time, try to protect your ears. First turn on the unit, then put the earphones on the unit with the volume knob at zero, and then put the earphones to your ears. Then start to raise the knob. In my case I raise it very carefully and very rarely over 30%. If you wear the earphones and turn on the unit you will hear a clip. There's no reason for this to happen.