I'd go with a Metrum Amethyst, especially if you can find one used. It's their entry-level R2R NOS DAC, but a good amount of proprietary technology is trickled down from their much pricier models. I'd think it would be a significant step up from your amp's internal DAC.
https://www.ebay.com/itm/123026758084?ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F7...
Here's a review FYI...
http://www.6moons.com/audioreviews2/metrum5/1.html
Best of luck.
https://www.ebay.com/itm/123026758084?ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F7...
Here's a review FYI...
http://www.6moons.com/audioreviews2/metrum5/1.html
Best of luck.