Sub $3500 range? That's a rather broad price limitation.... tons of good stuff there. Auralic Vega, NAD M51, Anedio D2, Yulong DA8, BMC PureDAC, Matrix X-Sabre, Questyle CAS192D..... and tons more I'm sure.
MacMini to DA8 to HDVA600 to HD800? That's tough because I've never heard the HDVA600 before. People who I trust have mixed opinions about it.... some like it, some don't. MacMini can be a nice transport if you run Audirvana or some other good software (assuming you like dealing with OS-X - some people don't). DA8 and HD800 is already a great pairing though, even from the front panel headphone out.