Before I chose the Benchmark PRE in an attempt to maintain some semblance of professionalism in my office setup, I was leaning heavily towards Headroom's new Ultra DAC coupled with a GS-1 from Justin at Headamp.
Headroom's new DAC is an untested product but with the tech they have powering the thing(Sabre chip) and Headroom's reputation for building solid products, my guess is it is going to be the bee's knees.
The gs-1 would have no trouble driving your Denon's to their fullest abilities, though you would have to look elsewhere if you are expecting the amp to add it's own sound signature to the mix.
As far as choosing an all in one solution that will sound remarkably better than your current setup, I think you are already into the realm of diminishing returns.
I am more of a believer in synergy between components, than more money = better sound. Staying in the realm of separate components will give you more freedom to find the sound you are looking for.
I have read complaints about the dac you are using but haven't any experience with it, but that might be a place to start. If I was in your situation, I would order headroom's new dac to check it out. They have a 30 day return policy, so if you aren't hearing all the magic that "better" gear promises, there is always an escape plan.
Also, your headphones need current, so a nice solid state and/or hybrid amplifier will work best.
And if that sounds like too much work...
Get the grace, it's headphone amp is most suited to power your phones and realistically speaking, the differences between it's dac and the [insert highly regarded dac here] would require weeks of careful listening and thoughtful consideration, or one solid dose of placebo.