"I have been used to consider poetry as the food of love"

Orsino and Viola
Public DomainOrsino and Viola - Credit: Frederick Richard Pickersgill
Mr Darcy is making rather free use of Shakespeare's Twelfth Night:

 

ORSINO

If music be the food of love, play on;
Give me excess of it, that, surfeiting,
The appetite may sicken, and so die.

Act I, Scene 1