As the Denon is a mid-fi AV receiver, I would look at DAC that also has a great preamp section. Otherwise you would have to worry about the extra cables you need in between and for power as well.
I may sound biased as I am an authorized dealer for T+A, but their DAC 200 is worth considering. But before I was, I compared this unit to a large range of DACs included a fully modded DirectStream, Lampizator Baltic 3 with $1K of tubes, a Mola Mola Tambaqui, Topping D90 MQA, Weiss 501, the Aurender A15’s built in DAC, and the $38,500 T+A SDV 3100 HV. The DAC 200 shares many similarities tonally with the reference SDV in terms of musicality, warmth, and detail, and it pairs a very good preamp with it (albeit only one set on unbalanced analog inputs). HDMI is an optional add-on if you need it. It’s a great unit for the price considering that won’t break the bank like the Tambaqui, and it may leave you with some money to invest in an amp, especially if you can sell the Denon and Monolith and recoup some cash there.