I personally think 666 was just the ultimate example of the finitude of man, and of those that are not of heaven. 7 is a holy number (seen all the time in Revelation), but it is also a number of completeness, or infinity ("How many times should I forgive someone? Seventy times Seven"). Therefore, 6 then is one mark short of perfection, and of the finite world.
Finite things cannot contain the infinite nature of God. The number 6 is a finite number that is short of perfection, and can relate to the finite concept of evil. Evil takes a finite number 6 and thrice gives it an attempt at perfection in 666. Is this so uncommon of evil? Evil also takes finite things and tries to make them infinite: it’s called idolatry. The number seven is a perfect number that expresses the eternal nature of God. The presence of numbers in revelatory experiences symbolizes God’s authority over all things God created.
Therefore, I do not believe that 666 refers to an individual or a thing, but rather the concept of God being full and complete, and those things that fall short of being able to worship or replicate God's completeness as false idols and a stumbling block for the Christian lifestyle.