Just imagine a beast with a lid for a head and fins for feet, running around looking as insane as possible (really imagine it!). When it runs up to you, it shows you the top of its lid-head. On it is written the numbers 666. Why? Because it's a spawn of the devil, and the number six is the devil's number.
Whenever we need to remember the reading ろく we just use the word rock.
So, think about the devil and the devil's number. What is often associated with those things? Why, rock (ろく) and roll is. Imagine the lid-head fin-legged beast suddenly blasting out rock music in the middle of his destructive mayhem.