Setting amplifier gains with o. Scope

[live2fish88]

Hey guys. Just purchased my first o. Scope,  an open VDS3102. I want to set my amplifier gains with a 40hz test tone. I need to measure the clipping point first of the head unit. How do I hook up the scope to the rcas to get a signal? Just as I would with a meter in a 12v cigg Jack? One on outside shroud, one on center pin? Next I need to measure at the speaker inputs. It's a monoblock class d 1400 watt 1 ohm stable amp. Anything else I should check or know?

Can a mod move this to the beginner section please?