Correct me if I'm wrong, but aren't you supposed to evaluate stacked exponents from the top down? That is, 9^(9^9) instead of (9^9)^9. If that's the case, 9^(9^9) is much larger than 9!!. Though I'm not sure how much larger, since I couldn't find any big integer calculators online that would give me an actual result for 9^(9^9).