Should I buy a seperate dac?

I have a Denon from the early to mid 90s.  I added a cheap Topping E30 to about 3 years ago, and was surprised at how much better I like the DAC.  There are better ones too.  

I would test if out with Dacs that you have a trial period with and decide if it is worth it. 

It’s a good bet the Bitfrost would provide a meaningful improvement, but don’t skimp on the digital interconnect as that can be a significant bottleneck to performance if you go cheap. Another DAC to consider would be the new Denafrips Ares 15th although it’s about $300 more expensive.  Best of luck.