I have to quesion the worthy of spending this kind of money to do this mod. The power supply mid and capacitor upgrade cost no More than 300 bucks.Gotta read my review in the signature link - pics of the massive tube rectified PSU all over the place!
But yes, the mods are only intended for using the device as your player. Adding an external DAC makes no sense in that case. It's one or the other.
If i want to do tube stage after the dac analog outputs, i would taking that balance pin out and build a special tube preamp for that in a separate enclosure, with its own power supply.