The other half of amp matching, and the half nobody calculates. A powerful amp is useless with sensitive IEMs if you can hear it hissing between tracks. Enter your headphone's specs and your amp's noise figure to find out whether that background hiss will be audible before you buy.
The figure is the sound pressure level of your amplifier's own noise, delivered into your headphones with no music playing. Because a sealed IEM blocks much of the room, your effective noise floor while wearing one can be as low as 15–20 dB SPL, so hiss around that level starts intruding on quiet passages. Over-ear headphones leak more room noise in and are usually far less sensitive, which is why hiss complaints come almost entirely from IEM owners.
Typical output noise figures: a well-measured desktop amp sits at 1–2 µV, a good dongle around 1–3 µV, an older DAP or a high-gain setting 5–20 µV, and tube amps can run into the hundreds. Note that gain matters as much as the amp: the same amp on high gain multiplies its own noise, which is why a low-gain switch is often the whole fix.
Two practical notes. First, if an amp only quotes a signal-to-noise ratio rather than an absolute noise figure, you cannot use it here directly — SNR is relative to full output, so a powerful amp can post an excellent SNR and still hiss badly into IEMs. Second, an impedance adapter reduces hiss, but it reduces signal by the same amount, so it only helps if your amp has volume to spare.