It is an unfortunate mix up as you have to wait for the MMCX cable. Sometimes good things take some time and I hope you have a lot of pleasure out of your I3 when you eventually get your cable.
In relation to the E1DA, I would suggest you give the BTR5 a chance. The reason I say so is that in this hobby we spend a lot of money chasing things which we find we already have in one form or other, so my view is, get a music player app, for example, PowerAmp (free for a trial period) or Neutron or both. Play around with the app, use the equaliser and get the sound you like. You can reduce the bass on your earphones increase the treble etc. The option of an equaliser on a free (for a limited time) is cheaper than buying the E1DA immediately.
As you may find, the deeper you get into this hobby, the more we collect things which we hardly use. The simple fact is you will prefer one of the two dac/amps and end up, mostly or only, using the one you prefer. You are better off finding the sound you like (although it may change with time) before buying more.