You don't listen to your subs, you stick your ear near your amplifier. That clears any confusion on how to listen for CLIPPING.
Before you listen to your specific test tone make sure you have all the speaker wires are disconnected from amp being tested and RCA's disconnected from the other amps (multiple amps), your objective is to see what the test tone does and not listen to what it can do. That said once you have everything hooked up and playing, check your oscope for a change from a sine wave to a square wave and your DD1 for a distortion red light to come on and back off. Listen to your amp real close and try to detect a faint buzzing noise (start of clipping) up to a point you can hear (hard clipping) it easily. I'm not sure if all amps do the same thing but environment should be really quiet in case of other confusing sound. The louder the buzz the harder the clipping.
