Plus 1 on this. I have a Washburn XB500, and put some flatwounds on it when I first had it set up. It was dull and lifeless, and I rarely picked it up to play. I preferred a 32" scale 5 string acoustic bass.
However, I needed to use an electric bass for a gig, tuned up, snapped a string, and decided to replace the whole set with Dunlop steels (40 to 120). Now, it is my main bass - wonderful instrument, lively, responsive, and exciting to play. The acoustic bass is now for sale (but I already have another - see below).
To select the strings, I went into The Gallery, and played a few electric basses, until I found one that I liked the sound and feel of. I asked "what strings do I need to make my Washburn sound like this bass here?". "Er . . . that looks like a set of Dunlop steels". So, for me, it was the strings - and not the bass - that needed changing.
I have a Boulder Creek 5 string now, fitted with a set of AB345 strings (phosphor bronze round wound winding around a nylon core), which I had not played for a while. When I returned to it, it sounded dreadful - but that was because I was playing with the same technique as I used on the electric bass.
It took me a while to adjust, but once I had persisted, and learnt the instrument (rather than the bassline), I was rewarded with a rich, musical tone.
So, perhaps changing the strings, and persisting with that instrument exclusively for a week or two, will yield a result.
Let us know how you get on!