Hope it's not too late to reply. Using DAP to stack with DAC amp, DAP only act as a source in providing digital files. However, the quality of digital out does affect the sound quality and sound signature as well. So different players pairing with Mojo will actually give you different sound.

This is also the reason that you can find very expensive media players that people use in their HiFi system, mainly providing digital source only.

So far from my findings, players digital out is definitely better than hand phone (micro usb otg / iphone lightning)
In terms of player digital out quality, I would rank it this way. (best rank first)
1) Cayin N6 (provide most details and good separation. Sound is solid and could be aggressive to some) -coaxial
2) Sony ZX2 (It's comparable with N6, with softer and smoother sound, more musical and very easy to control)- WM port -micro usb
3)iBasso DX80, DX90, DX50 (iBasso player actually has very good digital out quality, except DX80 is a little bit warm sound. For those who likes cleaner sound may prefer X7 over DX80) -coaxial
4) Fiio X7 (Quite clean and it's android base and support music streaming. UI is not as fluent as ZX2 as a source)
5) Fiio X5, Cayin N5 - coaxial
6) Fiio X3II - coaxial

Not sure if you are referring to the audio quality (DAC/amp) of hp & DAP, or just the digital out quality.
To me, the former brings huge difference. The good quality of DAC amp in a DAP provides us good sound quality. This is also why audiophiles would upgrade to headfi quality DAC amp from onboard sound card and handphone.

The latter is more for audiophiles who find that DAP can no longer satisfy them and prefer to stack up with a better quality DAC amp, OR they want a better quality digital source than a handphone.

Of course, whether the difference is huge, it is really depends and it's subjective.