It really depends on your headphones.
If you are using IEM's sub $150, I wouldn't bother upgrading to O2/ODAC or MM bundle.
Well I have following IEMs: Sony MH1C, NuForce Ne700x, Monster Gratitude, VSonic GR01, Musical Fidelity EB50, Klipsch X10i, Phonak Audeo PFE111, Sony XBA-H3, Philips Fidelio S2 (I just like to try different stuff ). Besides Musical Fidelity they all sounds great with E18. Musical Fidelity actually sounds better directly from HTC One or Sansa Clip Zip (don't know why - maybe because of it's neutrality and very flat response).
By MM bundle you meant Schiit?