TL;DR: EarAudio adapter is great, if you need balanced, go for it, its worth a shot.
Wanted to use my sennheiser HD660S with the balanced port of fiio btr5. I do use senns with ifi hip dac as my main driver, so just wanted to try the fiio balanced option. So, needed a converter which could convert 4.5mm male end of senns cable to 2.5mm male end (fiio has 2.5mm female input end for balanced connection).
Senns do come with 3.5mm regular cable so I had already used the fiio btr5 with senns (on 3.5mm) and was quite impressed with the output, but was still curious about balanced connection, so got the earaudio converter to give balanced a try and it was worth it.
The connector / converter (EarAudio 2.5mm male to 4.4mm female Adapter Cable) is of great quality, I don't think there's any loss in quality at all, at least while using with stock senns cable (which are already quite good). Serves the purpose well, build is great as well.
So, for someone who'd want a couple words about balanced connection, it's a hobby guys. For me, balanced (on fiio) sounded way better than single ended because fiio btr5 provides more power output in its balanced output than its single ended connection and secondly, btr5 balanced output is just way better in every way than its 3.5mm counterpart.
I used the adapter with senns hd660s, fiio fh3 (4.4mm balanced cable). For iems, its not that exciting to go balanced route as iem (in general) don't need that much power but if you have a full size headphone lying around which could use balanced, its worth a shot to try balanced (with something like btr5).
Caution: ifi hip dac also has balanced, but its balanced is useless for iem as it hisses a lot with iem, so before going balanced, make sure to read some reviews.
As for my iem, FH3 (with btr5), I'm using them with a balanced 4.4m cable which just changed the way it sounded originally. Fh3 with 4.4mm connection is a different beast, punches above its weight and in this particular case, the balanced connection was super worth it.