An adjective or (pro)noun for “more” along with an adverbial form ambë derived from √AMA “addition, increase, plus” (PE17/91). These words were part of a (brief?) conceptual framework around 1967 in which the intensive prefix am- was derived from this root. Elsewhere the root √AM generally meant “up”, and Tolkien seems to have reverted to an-intensives by 1969 as evidenced by anírime “most beautiful” (PE23/133).
Neo-Quenya: Despite these conceptual changes, I think it is worth retaining amba and ambe “more”, perhaps based on ✱an-bā and ✱an-bē. The first of these would clash with amba “up”, but I think it could usually be distinguished by context: á anta amba nin “give more to me” vs. mennes amba i oron “he went up the mountain”. See √AMA for more discussion.
ambë adv. "more", "used of any kind of measurement spatial, temporal, or quantitative" (PE17:91). As noun or adjective, amba.