Final answer:
The principal center for polychoral singing during the late sixteenth century was Venice, which had emerged as a significant hub for music and culture during the renaissance period.
Step-by-step explanation:
During the late sixteenth century, the principal center for polychoral singing was Venice. This period saw a flourishing of music as the invention of the Gutenberg press facilitated the widespread distribution of music theory and compositions. Venice, known for its unique cultural status as referenced by Petrarch when calling it a mundus alter or 'another world', became a notable center for musical activity. The influence of northern musicians who were in high demand for their training in polyphony significantly contributed to Venice's prominence in this era. The city's vibrant musical scene laid the groundwork for the development of opera in nearby Florence and the broader evolution of renaissance musical forms.