UA-1G crackling/peaking/clipping won't go away

Hello! Just got a UA-1G, which I am going to use for recording guitar (when I get it to work properly). I have some problems with crackling/peaking when I play hard on the strings. It just won't go away. 

I first thought it was a problem with the software (using Ableton Live), but I realized that I get the same crackling when I turn on the monitor. I even tried to take the input knob all the way down to the lowest, still audible, setting and the crackling is still there.

When I play the strings carefully, or play mostly on the upper strings, then the clean sound is fine (although maybe a little muffled, but that can be a guitar string issue since I play on kind of old strings), but as soon as I play a little harder, especially on the lower strings, then the crackling is almost constant. 

I've tried it on a drop C-tuned guitar with a .012-string set, and a open E-tuned guitar with a .010-string set, and it's the same with both, so don't think it's something wrong with the guitar. I don't get any crackling when I play through an amp anyway.

I plug the guitar directly into the UA-1G