Hi,
First of all we put a BIG RED header above the question field asking for information that is usually necessary for answering the questions... In your case we know you have an AudioboxUSB, but not what your using for a computer...The reason we say this is because often laptops have USB ports that are not all equal, and often require a POWERED HUB to provide sufficient power to operate the AB USB. The AD USB is a port specific device, meaning it MUST be plugged int the SAME PORT it was installed on, and, if you want to try a different port you need to uninstall it first.
There is a wealth of information in the Knowledge Base:
http://support.presonus.com/forums
and under the Audio Box heading you'll find the uninstall process and some things people have asked about in the past...
Now, another problem with your post is that you didn't tell us if your plugged into the computer to monitor the audio, or the AB USB itself...OR if it's the playback from a recording of the instruments, or your monitoring the guitars input sound...
If you are plugged into either input, and you have adjusted the gain for that channel so there is NO CLIP LIGHT, and you have the MIX knob fully COUNTER clockwise, you should NOT have distortion.. HOWEVER... due to impedance differences in various sources (like guitar or microphone, even Active and passive acoustic instruments, you MAY need an interface/signal matching device such as a direct box to get the BEST sound at the recording.
Note also, that if the sound your talking about is the recording, and your computer has minimum specs or below... your computer might need some help like optimization for digital recording. These too are located in the Knowledge base...
