By the way, the answer to your first question: Clipping is possible at any gain setting. If you put enough signal into the front of the amp at the RCA inputs, you can run the amplifier's output devices to their maximum amplitude at the minimum gain setting.
Clipping distortion, particularly the clipping an amplifier does when it runs out of headroom usually causes fuzzy, crunchy, crackly sound... A good example is junior driving around in Dad's car, cranking the living daylights out of the stock stereo system, all nasty and distorted.. We have all heard that car go by, most of us laughed at it, and a significant number of us have actually done that to the stereo in Dad's car...
The problem is, I still hear a lot of badly distorting systems when I am out on the water here and there, and I think it is due to the fact that a lot of people grew up listening to badly distorted audio systems and do not know the difference between clean and distorted audio reproduction.
The best way to know you are clipping is to listen for it and learn /know what it sounds like. A goofy example, but one that will allow you to listen for and understand what it sounds like would be to play something that is totally acoustic. A clarinet recording; a violin recording; a spoken word recording.... We hear un-distorted speech every day, and to hear that going into distortion should be easy. Take one of those recordings and play it. turn the amplifier gain up... up... way up until it starts sounding different.. Crunchy, starting to get harmonics, fuzzy, ragged... These are all vague descriptors I have to describe the sound, but you might come up with different ones. The point is, when you are cranking things up so much that they quit being just louder, and start to sound different; that is a real good clue that you are clipping... After that, you could use some more complex music, and go through the same exercise. Listen for that point where the clean sound goes away; listen for the point where it quits getting louder, listen for that crunchiness to start taking over the sound... That will be clipping....
If we all had oscilloscopes it would be easy. We could look at the signal at any stage in the audio system. At the point where the nice roller-coaster hills and valleys start having flat spots at their tops and bottoms is the place where we run out of headroom and start clipping. Without the benefit of oscilloscopes, we need to rely on our ears....
Phil
Kicker