Here's my take on using the Onkyo DP-X1 with or without external DAC/amp combos:
If, and only if, you never, ever, want to use balanced mode, or you only own single-ended headphones (unmodded, unmodifiable, or never getting balanced cables) get Pioneer XDP-100R, and maybe something like the Chord Mojo.
Ditto Questyle QP1R, Echobox Explorer, etc.
If you wish to experiment with balanced and single-ended output plus the benefits of streaming and high storage capacity, then consider DP-X1.
If you pick the latter, but you have some single-ended headphones that you wish to max out, then DP-X1 + Mojo is awesome.
So, in my case, if I wanted to max my SE AKG K 712, Meze 11 Neo, or KEF Audio M500, I would do exactly what
@masterpfa has done and get a Mojo to play with DP-X1.
However, I do not want to do this personally, and my slightly modded Meze 99 Classics and Audeze EL-8 Titanium have balanced cables for balanced playback with DP-X1.
I should add, it is entirely possible that DP-X1 + Mojo SE output is better than DP-X1 balanced (either BTL or, to me, the #Orpheus superior ACG mode).
If I wanted maximum transportable balanced system, then DP-X1 + ALO Continental Dual Mono would be a fine (but hot) system.
Furthermore, I do not want to carry a stack, but if Onkyo released a professional add-on for DP-X1 for more power and battery with balanced out (like Astell & Kern AK3xx series), that would be a serious consideration.
So, DP-X1 with Mojo is fine, especially for future proofing oneself and have both SE and balanced options.