I abandoned the idea of going for the Fosi Audio V3 mono amplifiers and pairing them with the Wiim Ultra. At the moment I can say that I like the NAD sound more. My idea is to go for this setup: NAD C355bee (class a/b amplifier), Wiim Ultra and possibly an external DAC. My speakers are the Dali Ikon 1 mk1. I bought the NAD C355bee yesterday cheaply second-hand.
I still have a Wiim Mini and SMSL SU-1 DAC lying around that I will return before the 14 days return period. I initially tested the NAD C355bee with the Wiim Mini. That sounded pretty good. Then I also added the SMSL SU-1 DAC. In my opinion, it sounded different then, a bit more airy, improved soundstage, bit more detailed and placement of all elements of the sound was a bit better. Only now I think the sound sounds a bit too bright. I like a little more warmth/smoothness.
Could a (different) external DAC give more warmth/smoothness to the sound without sacrificing the clarity and definition of the highs and mids in case I also find the sound of the Wiim Ultra + NAD C355bee too bright?