In these matters one must distinguish between nature and ability. Fallen angels have gone from +999 to +666 as to nature and to -666 as to the motivation of their nature. But as to ability they might very well be a billion. So with Satan himself. I know I am very much speculating, but I wonder whether Solomon at the nadir of his fall attempted to turn his fallen nature of +666 around to +999 and whether he thought he could progress to +1000 to become Messiah. However it is not possible to go from a 6 to a 9. From a 9 or 7 to a 6 is possible when one's very nature is not an 8. By the grace of God he was restored towards the end of his life and it was granted him to foresee the Messiah, much like the patriarch Abraham saw the Day of Christ (the millennium) and rejoiced about it.
Also ancient cultures like the Greek one understood that +777 meant the fullness of human perfection. It is strange therefore that Nebuchadnezar did not apply the number 7 to his statue as described in Daniel 3. Unless he was humble and realistic enough to know that fallen man by himself cannot advance to 7. Therefore he must have reasoned that it is humanly impossible to turn a 6 into a 7.
Fortunately for born again Christians it is possible to grow to perfection (as the apostle Paul said:
Let us advance to perfection and this if the grace of God permits it). And if they do not, then they will reach perfection at any rate at the resurrection; that is more or less. For after the Judgment Seat of Christ they still will have to experience the bad done in the body and not just the good. Because the born again nature is +888, perfection is possible. And not only that, to all eternity the saved will continue to grow on in the Lord God. For God is infinite and we are finite and therefore eternal growth is not only possible, it simply will happen. Even in hell there will be advancement in understanding, but often like in an unwilling school and in heaven it will all be playful and natural.
The beast will have a negative motivation of -666, or even -777. But his nature will also have a positive side of +666. This denotes his spirit, soul and body. His abilities will be very advanced, so much that he can perform great miracles. As said before, the triple character of his nature, also points to his claims to divinity. And so the antichrist will be able to make fire descend from heaven.