Regarding your question about kirtana, practically we are not concerned with the instruments. They are used sometimes to make it sweeter, but if we divert our attention for using the instruments more, that is not good. Generally kirtana is performed with mrdanga and karatalas, but if somebody is expert instrument player, he can be admitted to join Sankirtana. We can accept everything for Krishna's service, but not taking the risk of diverting attention to any other thing which will hinder our Krishna Consciousness. That should be our motto, or principle.
Regarding your question about kirtana...
Correspondence
1969 Correspondence
We are not concerned with the instruments. They are used sometimes to make it sweeter, but if we divert our attention for using the instruments more, that is not good. Generally kirtana is performed with mrdanga and karatalas, but if somebody is expert instrument player, he can be admitted to join Sankirtana.
Letter to Jadurani -- New Vrindaban 26 May, 1969: