If money is an issue, you can get Audirvana for $79.00 It is a vast improvement on the built in DAC.
Either i'm missing something here or need to point out an error. Audirvana is a media player (software). It is not a hardware digital-to-analog converter. If you use Audirvana (or itunes or any other player) you still need a DAC to physically create the analog wave-form from the code. The MBP has a cheap one built in. You can buy external, better ones from $100 -->> $10,000, with diminishing marginal returns.
Someone also asked about the superdrive. In general any such removable, optical media, compared to playing the ripped file off the HDD or SSD (better) will generate higher noise at minimum. Whether you find this significant depends on many factors that i wont get into.