Turn your PC volume down to 5% and the volume knob up to 100% on most amps will instantly result in obvious clipping. Almost every amp I know of clips at some point due to low input amplitude.
Clipping, by definition, is caused by too high amplitude (not gain, not volume knob position, but an actual signal level that is too high). If an amp clips under the conditions you described, it is because it has excess gain even with the PC volume turned down so much (unlikely). Increasing the input voltage does not increase the output clipping voltage (in other words, you cannot magically turn an amp capable of 50 W maximum power output without clipping into a 200 W one by feeding it twice as high input voltage).
The volume knob controls the signal output last time I checked via attenuating the input voltage. If you do not have the power the signal clips.
The signal output always depends on the level of the input signal, in addition to the volume control, and the gain. The whole reason why you need to turn up the volume is that the input level is too low. If, for example, the amp clips at 20 Vrms output voltage, and has 26 dB gain, then either a 2 Vrms input at -6 dB volume, or a 1 Vrms input at full volume would just reach that level; the first case has 6 dB of excess gain. With a 0.5 Vrms input and maximum volume, it would simply output 10 Vrms without clipping; in this case, there is not enough gain to reach the clipping level at all.
Too high input level, on the other hand,
can cause clipping in some amplifiers, and when that happens, it does not even depend on the volume setting on the amp. Examples include the FiiO E9 (it clips a 2.1 Vrms input), and the O2, depending on its gain setting.
