The problem with the impedance ratio when it comes to iems, as I see it, is that most (all?) iems have much lower impedance than do full headphones. So you are never (or almost never) going to get that sweet spot ratio of 8:1 or 10:1 (depending on who's talking). This includes iems like the Roxanne and the 846, but also the Altone200 and the FidueA83. So, if you can't get the ratio, you are left with knowing that any iem you get will be, on paper, "wrong."
At that point it comes down to sound. I'd suggest throwing out the paper and going by other users' comments. Since the AK series has an impedance somewhat close to the Pono, and both sound good with the 846, listen to the owners. While an X3 is (from what I'm told) good, I don't think it compares to an AK anything or a Pono. Getting it solely because of a ratio seems counter-intuitive. The great thing about this site is that you can ask others what they use. Look at the sigs on this thread and see what folks pair up with their 846s. Or simply ask. I can tell you that the 846 sounds good with the X5, and it sounds better (to me) with the Pono. It sounds even better still with a balanced cable with the Pono. Lots of folks here use AK and DX90s, see what they say. Cayin has a nice new DAP that's getting good reviews. That can be a good starting point.