I do like my Topping DX7s......but.
You might consider to keep using the Asus Xonar DX as a DAC and connect an external headphone amplifier to it.
A head amp that also comes with a separate line-output, for the studio monitors.
JDS Atom or Schiit Magni headphone amplifier can both be connected to the front speaker/headphone jack of the Xonar DX
and come with a line-output.