Do I need a DAC with an Amp?

I really need to get a DAC, but I'm not sure if I need one with an amp.

My current setup is my computer going into my Marantz 1530 Amp (1978), and that goes to my bookshelf / ATH M40x headphones.

I need to get a DAC because the onboard sound is awful and I don't know if my current amp is compatible with one (or is new enough).

Thanks.

Edit: Here's a link to the Marantz http://www.vintage-audio.com.ua/en/cat/334/1538.html