I have
mentioned once before on post #748 of this thread, that for the CK100PRO's in terms of synergy with single DAP setups I have enjoyed them pretty much with the AK120, as well as my older NW-HD5 by Sony.
For stack-fi portable DAC/amp combos if u happen to be using an Apple player, you may look into Centrance Hifi-M8 or Fostex HP-P1 as these are certainly very gd to my ears as well, albeit it comes with extra bulk/portability issues which would put some pple off easily like me so I'm all for single setups.
Have no idea how the iBasso DX50 stacks up against the X3 or X5, but a bro in here did owned the X3 before moving on to the DX50, and according to what he had shared with me previously, the DX50 was quite a noticeable improvement above the X3 although he also sold off his DX50 set eventually later.
If you are interested, the Sony Walkman NWZ-ZX1 will be launching here officially in less than 2-3 weeks time (Taiwan's release date is officially on the 2/14) and I'll be getting it immediately upon release, and since I still have my CK100PRO ard with me I don't mind sharing some impressions once I can get some proper listening sessions with this combo.