The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
16. And scripture declares a difference (in the case of meditations on symbols).
With reference to the meditations on symbols, such as name and so on, scripture declares that each following meditation has a different result from the preceding one, 'As far as name reaches he is lord and master;--speech is greater than name;--as far as speech reaches he is lord and master;--mind is greater than speech' (Kh. Up. VII, 1, ff.).
[paragraph continues] Now this distinction of rewards is possible because the meditations depend on symbols, while there could be no such distinction if they depended on the one non-different Brahman.--Hence those who take their stand on symbols cannot have the same reward as others.