Impedance adapters and frequency response on not overly sensitive IEMs

[carpman]

I posted this on a thread related specifically to the Xduoo X2S, but this is a rather tangential question and felt it should be in its own thread. So, I'll link from that thread to this one to stop anyone answering in the wrong place.

I wanted to see the effect of a relatively low resistance impedance adapter on player volume and SNR (basically to reduce hiss) - as you can see in the context quoted below, I didn't expect such effects on the musical content (FR).

What I'm trying to figure is really this (from below):

So what I don't understand about the frequency alterations, when I look at the impedance graph on the Tanchjim One review, it's just flat. How much is to do with the source (X2S impedance, 0.5 ohms) and how much is to do with the recipient (the IEM, in this case 21 ohms measured by Amir, with a stated spec sensitivity of 126dB/Vrms) + 18 Ohms additional resistance via the adapter between the player and the IEM.

CONTEXT (thread source)

The Xduoo X2S for some sensitive low impedence IEMs has a hiss issue. For me, even though my stuff isn't super sensitive, on the Tanchjim One and 7hz Zero 2 I can make out some very minor background noise, but only between tracks and it's really not an issue. But I was interested in the effect of impedance adapters and whether they'd actually work to increase the player volume and thus increase the SNR and lower the hiss.

I also watched this Youtube video on Impedence Adapters on AudioTechTalk. I got a couple of these JCALLY adapters just to see the effect (18 ohm and 36 ohm). The impression from the Andy guy on the video is that anything around 25 ohms is highly unlikely to have an effect on the FR. That was not my experience.

I put the 18 ohm adapter between the X2S (which a reviewer measured at 0.5 ohms):

... and my Tanchjim One IEM. The audible effect on music was approximately that is sounded like someone had done a Mid-Side DSP thing and pulled down the volume of the mid channel recessing the vocals and other central material and then applied a slight roll-off to the highs and a noticeable bass boost (of perhaps, just a harder pull down of the higher frequencies that sounded like a bass boost. So it was boomier with recessed vocals and mid channel mid frequency content.

At least that's what it sounded like.

So what I don't understand about the frequency alterations, when I look at the impedance graph on the Tanchjim One review, it's just flat. How much is to do with the source (X2S impedance, 0.5 ohms) and how much is to do with the recipient (the IEM, in this case 21 ohms measured by Amir, with a stated spec sensitivity of 126dB/Vrms) + 18 Ohms additional resistance via the adapter between the player and the IEM.

I've watched quite a few videos and read quite a bit on ASR and elsewhere, but I'm clearly not getting what is doing what and under what conditions does this effect frequency.

This is not currently a practical issue for me, as I'm happy with the player (the minor hiss between tracks at a volume of 13/30) and both my budget IEMs, but I was surprised about the effect on the musical content. To such a degree I didn't even focus on the hiss between tracks.

Anyone who can explain this to a dumb-ass, I'd really appreciate it.

So, any help would be greatly appreciated as to what's doing what. Just for my understanding, as I said, the player noise is inaudible to me once music is actually playing.

Thanks,

C.