Possible Clues:

• Six performing together
• Group adds short extension to their standard musical numbers
• Group of half a dozen musicians ( X, not S )
• Large chamber group
• Brahms' Opus 18, for one
• Male and female, it seems, in the group (6)
• The friends on 'Friends,' e.g
• Moderate-sized chamber group
• Set of six texts put out around the East
• Partly half a dozen for male and female, by the sound of it