Either 64udio/ADEL or majority of other multi-BA iems/ciems will not benefit significantly from amping, especially double amping when you amplify sub-par HO of the phone, unless you are looking into a specific amp to color the sound or to take advantage of integrated bass boost. If you want to take your iems to the next level in terms of sound quality, you definitely want to look into dac/amp combo to improve the resolution of the sound, support higher sampling rates, maybe to expand the soundstage, and to take advantage of external amping with a clean filtering taken outside of your phone shell where everything is cramped and analog/digital circuits are mixed together with poor partitioning (pwb of the phone is just too dense and not enough room for discrete filter caps or proper partitioning and shielding of wireless radios).
I tested a bunch of small usb dacs in the past, they offered advantage when used with my old Note 2, but not as much as with my current Note 4 (and I have no plans to upgrade yet since i perfectly fine with my phone, especially quality of the camera which got worth in N5...). Where I'm getting at, sub $100 is not going to offer your too much advantage to take you to a noticeably higher next level. DF is a fine product (original or black or red), but it's leaning toward the smoother warmer side which in my subjective opinion is not the best for U6/U12 (not sure which 64Audio iem you got?) if you want to improve the resolution, layering, and to bring up more details. Thus, you need to step up in your budget. I haven't tested it, but heard from a few reviewers whose opinion I trust, and they were very impressed with a slim Opus#11 dac/amp (seen it for $235, converted from GPB), especially how well it integrates natively with Android.