It’s a pretty good pairing, and the end result is slightly better than the Triple Driver “Lightning” Model - which I think has much more to do with the IEM itself rather than the respective DAC/amp parts.
The biggest advantages of the FiiO i1 in this case are a) the extra cable length and b) the ability to run other IEMs with it when I want to (it pairs really quite well with the ER4-XR for example … very neutral combination).
In terms of comparative performance … I cannot realistically compare against the Lightning interface of the Triple drivers without buying a passive version as well. Which I might do at some point, but would rather wait and see if 1More come out with a Lightning version of the quad drivers (and made the cable a foot longer).
I will say that things like the AudioQuest Dragonfly Red perform better than the i1, both in objective terms such as supported resolutions and raw power (the i1 is 30mw @16 ohms), and also in terms of raw sound quality … with the i1 having a slight treble tilt and the DFR being very slightly tilted the other way (which tends to work better with IEMs) but the i1 is also a lot more convenient in use and uses much less power (at the cost of not really being suitable for “full-size” headphones).
A very basic ordering would, perhaps unsurprisingly, put things this way:
Dragonfly Red > Dragonfly Black > FiiO i1 > Apple Lightning-to-3.5mm adapter.