miceblue: those frequency response variations are typical of high output impedance combined with non-flat impedance curve from the IEMs, which is very common with BA IEMs. To avoid those variations (which distort the audio), you want to plug them into a headphone out with an output impedance...

Actually, voltage requirements being equal, lower impedance headphones are harder to drive, because: they require more current than higher impedance headphones they require smaller output impedance in order to be driven properly (e.g. 18 / 8 = 2.25Ω), which limits the number of suitable...

Shure SE425, and Bose QC15 headphones

Not that my Clip+ or headphones or IEMs need an amp, but it's a nice amp! (Topping NX1)

What standard? The X5 is rated at 1.5 Vrms, the X3 at 1.7 Vrms; "Consumer audio" is about 0.3 Vrms, "professional audio" is about 1.2 Vrms, a CD player is 2 Vrms, an iPod Classic is about 0.83 Vrms, a Clip+ is about 0.5 Vrms, and my Sony Xperia Z1 smartphone does about 0.13 Vrms. As you can see...

He did already:

With the iPod's output impedance of 5.5Ω, ideally, you want headphones / IEMs with impedance that is at least 8 times higher, i.e. 44Ω. Your IEMs are rated at 47Ω (and they're fairly sensitive, too, at 106 dB/mW), so they're a perfect match.

Sorry, but you're doing it wrong. You complain about "quite loud hiss": this is why. High gain with those IEMs will make them sound worse, not better. I can only imagine the sheer effort required to push a button…

You definitely don't want to set the C5 to high gain with those IEMs, ever.

I don't understand the point of such ridiculously high sensitivity. It's more of a flaw than anything else.

Is the amp set to low gain? That's a ground loop. You can try a ground loop isolator (search on Amazon or whatever).

It still is an actual problem, yes. Many devices still have excessively high output impedance (10-100 ohms, like smartphones, tablets, computers, as well as other devices with a headphone out, such as active desktop speakers). Some devices also have very weak output (notably, certain...

There's a red LED at the back that lights up when charging, and turns off when it's done.

4) it saves a lot of bulk and cables and inconvenience of an external amp

"I think we'll just have to settle with turning the amp off if we want the rest of the package." I meant, why are you dismissing the built-in amplifier.