The problem is being greater means possessing more or being more of something. If Y is defined as lack of X then the greater being, the more X it has.
SO I think your objection is easily refutable. If you're interested here are my objections against TOA:
It's circular argument. God exists because god exists by definition. Some people try to avoid it by saying "no no, it is not so, we only uses a property of being MGB" But that's the problem, because your definition of MGB includes property of existing in all possible worlds. In other words, people who use TOA simply pretend to derive it from MGB property, even though it's obvious a thing is MGB only if possessing property of existing in every possible world.
So basically TOA comes down to "God exists, because god by definition exists"
There is another problem namely being MGB is self contradictory. For example god must be maximally wet and maximally hot at the same time. It's impossible. We can't even use the oldest trick and say that god is beyond logic in this case because it would mean god isn't maximally coherent logical structure. Also we clearly see that god isn't MGB for example god isn't maximaly visible, or maximally recognizable, or maximally proven.