Purity of consciousness depends upon purity of mind. Purity of mind depends on purity of actions. How you eat, sleep and whom you associate with matters a lot.
When something is pure it is natural. When something is contaminated it is unnatural. So purity of the heart means to bring us back to our natural state of loving God.
Pure things are powerful. Become pure and powerfully preach pure bhakti.
To do devotional service, to chant attentively, one has to be pure in mind. To be pure in mind, one has to be pure in action. To be pure in action, one has to be pure in what he eats and with whom he associates.
Due to previous samskaras, one may be expert in some kind of activities, but purity is a different thing.
The quality of chanting dpends on purity. Purity is dependent upon surrender. Chanting and Surrender happen simultaneously.