The DX3 Pro+ has more than enough power for either DT770/80 and HD6XX. However, its amplifier is not so good for low impedance loads like the Aeon Closed X (12.5 Ohm) or Edition XS (18 Ohm).
The DAC measures slightly worse than the DL200 should but the differences are almost certainly inaudible. Consider just getting a dedicated amplifier better suited for low impedance loads. However, since the DX3 Pro+ only has unbalanced RCAs for the line outs, noise from ground loop cannot be ruled out. Getting a DAC with balanced line outs to connect to the amplifier with balanced inputs would most likely eliminate that. Thus, you could consider the DL200 for that alone. However, the DL200 should perform about the same as the C200 and is thus much better suited for low impedance loads while still having enough voltage available for high impedance loads though less than the DX3 Pro+. Even, if you want an all-in-one unit, the DL200 may be better for you than the DX3 Pro+.