>>106233237
You need both a DAC (Digital-to-Analog Converter) and an amp (amplifier) for headphones with sufficiently high power demand. Must "plug-and-play" headphones are designed with very low power requirements and usually use the (inherently not great due to electromagnetic interference) DAC inside your computer. Both of these things lead to worse sound.

In my suggestion, the iFi Uno is an external DAC + amp combo unit you connect with USB. The DT 770 Pro is just the newer model, while the 990 is a different open-back (leaks sound, does not isolate you from surroundings, but will sound better/more open) pair of headphones. There's also the newer Pro X (48ohm) which costs a bit more and has a newer driver with wider dynamic range. You should get a DAC + amp for these headphones either way, the difference is very noticeable.