Greetings from China.
Heard a pair of these last week with a Shanling M6 2021 DAP. Not sure about whether this particular pair of FH5S was burned in, I was told it just arrived.

That pair was terminated with 3.5mm SE, so I did not try them out balanced.
I am quite familiar with FH5 too. Here’s my impression.
FH5 VS FH5S (impression of the FH5 from memory):
1. The FH5 has a W-curved sound signature, in that each of the treble, mid and high frequency bands seemed to work separately. The FH5S has a much more coherent sound, the transition of the bass to mids and mids to high is very good.
2. While the FH5 has a somewhat exciting signature. FH5S is tuned towards maturity and politeness. I am not sure driving them balanced will bring more life out of them, nor do I know whether they were well burned in. Looking at the difference in sensitivities and impedances of both, I have a feeling that the FH5S would probably benefit from balanced amp or with different settings on the switches. And I do believe I haven’t heard them in close to full potential. I would rate them as potentially great in SQ.
FD5 VS FH5S:
1. Both produce enormous soundstage, due to semi-open designed (But don’t expect to hear the environment much like AirPods or other earbuds). The presentation of the FD5 is front row, while the FH5S is like 5th row. The FD5 has a bass which is powerful and in your face (or in the head), the FH5S’ bass, mid and treble are more inline in one plane.
2. FD5 has a mild V-Curved FR, the FH5S is much more neutral.
3. The FH5S has tuning switches, the FD5 not. But unfortunately I haven’t tried out the different settings. All impression based on default setting
4. The FD5 has a unmistakably DD sound in the treble region, which could appeal to some.
5. The FD5 appears to have slightly more resolution, though I could be tricked to believe so given the more elevated treble.
6. The sound signature of both couldn’t be more different, Fiio wants us to buy both
That’s all I can think of now, hope that helps.