For everyone in a similar situation, here’s the last update:
The ifi DAC+AMP and the microphone arrived last week and the DAC+AMP basically eliminated the problem with the mids. Mids are now exactly as Falenkor stated, slightly recessed but not muddy.
The Samson microphone also works fine but I ended up ditching the InnoGear popfilter for the OEM one because it is more convenient.
As far as the ifi Zen DAC+AMP goes, it’s a bit sad that I have no option to connect my speakers to it and I had to connect the speakers to the back of my computer again. But that’s only a very small price to pay.
I always listen with the “Power Match” feature of the DAC+AMP turned on and the “TrueBass” feature is active most of the time.
I have also noticed that certain sounds, e.g. someone sneezing in voicechat, sometimes lead to the sound being very muddy and bugged. But deactivating the “TrueBass” feature and activating it again by pressing the button on the DAC+AMP solves this problem.
I suppose that this might be caused because the AMP does not get enough power since I have not connected it with an extra power cable but only with the OEM USB 3.0 cable for power. But I might try switching that out in the future for a separate power cable because the OEM USB 3.0 cable is extremely short anyway (roughly 50cm / 20 inches).