After reading carefully the lasts messages I want to add my own opinion an experience about strings and pick ups.
About using flatwounds on your Sandberg: it's up to you, what works for me doesn't necessarily means that work for everyone. You have to try it.
In my opinion the best match for a flatwound is a pbass (or vs in sandberg) for me is the configuration where fits better. I used Labella in my Enigma 5 and even when I feel comfortable and liked the sound I always felt a lack of....something, so I returned to rounds again.
At the moment I have never found a flatwound in 5 string version that I like for a long time.
In 4 strings my choice is very easy, Thomastik.
About changing pick ups, I'm not a very big fan of changing anything on my bass. Sometimes I have had the idea of adding a passive tone to my sandberg (Holger please do it!!!) but I refuse, I like my basses with their pros and cons.
If I have to change some pick ups looking for a vintage tone I have 3 options in no specific order: Lollar, Lindy Fralin and Kloppmann.