A.The AC-3 uses COSM technology to emulate the sound of an acoustic guitar, which has a complex overtone structure and a rich high-frequency content. To amplify this sound accurately, your playback system needs to be able to reproduce these high-frequency sounds. Unfortunately, a traditional guitar amp attenuates high frequencies, making it unsuitable for reproducing the sound of an acoustic guitar. Since a PA system has a flat, full-range frequency response, it's able to provide a natural playback sound that preserves the rich overtones and high-frequency range of the AC-3's acoustic guitar sound.
Therefore, to get the best acoustic guitar sound from the AC-3, you should connect its LINE OUT jack to a PA or other full-range amplification system.

If you connect the AC-3 to both a guitar amp and a PA system (or recorder), switching the AC-3 on/off will automatically switch the output between the LINE OUT and G.AMP OUT jacks. For details,
refer to the related FAQ:
When the AC-3's effect is on, I want to send its output to a PA system; when the AC-3 is bypassed, I want to send its output to a guitar amp. Is this possible?
The AC-3 has two output jacks: LINE OUT and G.AMP OUT. Which should I use, and how do they differ?G.AMP OUT jack
(2006/03/27)