You can play FLAC format from a USB stick directly plugged into the Denon dnp-720ae. You can't use the player as an independent DAC. Try the sibling Marantz NA7004 for that.
I too have looked for details of the Denon's internal DACs, but have unearthed nothing.
What I can say is that whatever components are in use, and I suspect they are of the BURR-BROWN variety, they are impressive, as is the dnp-720ae in most other respects.
I originally had the machine bypassing its internal DAC circuitry to feed an off board Arcam rDAC through an optical connection, but after some running in, was so impressed with its native sound that I decided to unleash the player and exploit it's own built in DACs. This works for me as I wanted to tame my KEF XQ30's slight brightness and experience more of that familiar, warm Denon sound. I am also using a high end Denon AV receiver which adds to that warmth. I can't testify that similar results will be had in other set ups, and remember the rDAC really comes into its stride when connected from to a computer using its USB connection; but pound for pound, you get a great sound from the Denon and would need to go some way up the DAC scale to improve on things.
The rDAC was sold in under 6 minutes on eBay.