I didn't use the 18db boost (cuz I believe it distorts the original tone, doesn't it????)
but then thanks to your post, I lowered the hardware monitor volume in the LINE 6 Audio-MIDI Devices control panel setting and the original problem is now fixed
however, can you reply it back to me more about +18db boost? It seems useful but then when I used it in my previous computer, I got the right volume, but then the tone seemed to change (in a way it's different from when the volume was low).