If you’re using gut strings and a realist pickup, that is the darkest, boomiest combination I can think of, although I’m certainly still learning. I would start by getting the acoustic sound you like the best, forgetting about amplification initially. If you liked the zyex, you could try zyex EA and gut DG. Forget the phosphor bronze idea, upright bass is not an acoustic guitar! Just experiment with strings until you find what like best or run out of money.
Then try different pick ups, those are actually cheaper than strings these days and there’s usually no trade off buying them used. I tried 5 different ones over the course of a couple of years. I settled on a Yamahiko single element. To compare them, you need an impedance matching preamp or a high impedence input. I personally have found that 1 megohm does not do the trick for most piezo; I.e. they sound better with a higher (10megohm on my Clarus) input. There are cheap impedence matching preamps around, the old fishman model B is one. It doesn’t have good eq options, but at least it eliminates the impedence mismatch that really kills a lot of piezos.