[mrconfuse]

Currently I have a Westone Umpro 10, a Ibasso D-zero mark 2 and my iphone . I can do 1 upgrade at this time. I nailed it down to the following items that i may want but now sure which is the best one to get at this time. 
 
Replace the iphone with IBASSO DX50
 
Replace the Westone Umpro 10 with Etymotic ER4SR or Ibasso It03
 
Replace the Ibasso D -zero mark 2 with Fiio E17K 

 

[serman005]

  Currently I have a Westone Umpro 10, a Ibasso D-zero mark 2 and my iphone . I can do 1 upgrade at this time. I nailed it down to the following items that i may want but now sure which is the best one to get at this time. 
 
Replace the iphone with IBASSO DX50
 
Replace the Westone Umpro 10 with Etymotic ER4SR or Ibasso It03
 
Replace the Ibasso D -zero mark 2 with Fiio E17K 

Generally speaking, the greatest improvement in a single move within a system comes with a headphone upgrade. That said, I really like the ER4SR. Have not yet heard the IT03, but it is getting good reviews. In this price range, I would definitely add the Campfire Audio Orion to the list. It is spectacular. Balanced, with superb bass and good treble extension without sibilance. A wide stage. Nice air. A really nice earphone. Worth reading about.

 

[ProtegeManiac]

  Replace the Ibasso D -zero mark 2 with Fiio E17K 

 
If your role is to upgrade I'm not sure why you'd consider a sidegrade. Just because the E17K costs more doesn't mean it has markedly more performance.
 

 
Replace the iphone with IBASSO DX50

 

Just note that people who use iDevices are typically the ones most likely to come back to the DAP threads to rant about how nothing works like iTunes.
 

 
Replace the Westone Umpro 10 with Etymotic ER4SR or Ibasso It03

 

I'd replace the earphone first, but look around for more options than those. Or actually save up more and pour more money into the earphones. They don't need a lot of power anyway and even then you don't mind using the iPhone with the D-Zero MkII.