There is a lot of win in this thread.
I do find the RE0 and RE-ZERO to be geared in a general sense the same way. Both are super clean with a very tightly controlled note and well balanced frequency response. Beyond that, they do start to differ a bit though. The RE-ZERO has a more effortless dynamic range, a little more bass presence, better sound stage, more balanced frequency response, and more natural sound, For $20, it does a lot of things better, not to mention working well off any low power player. The tightly controlled note tends to make it a little too constrained (not effortless, less dynamic breadth) and lean (less full-bodied presence, thick in note) than I personally like, but it's a darn good product and at least in my eyes a worthwhile step up from the RE0. While the RE-ZERO sounds a little lean, the RE0 sounded tiny and weak and really does require a rather robust amp to really drive it well. I think the RE0 had better texture/articulation of the notes, but it's been about 2 years since I've owned a pair. Despite the $20 difference in price for the RE-ZERO, not needing an amp to drive them is a money saver over the RE0. Even with a budget option like the FiiO E5, you're still paying more for the RE0 in the end.