I switched from the nickel round strings that came with my Ibanez GSR180 to Thomastik-Infeld JF344 flat strings.
I have tried two flat strings in the past. D’Addario medium gauge chrome flat strings (ECB82) and GHS stainless steel flat strings (M3050), neither of which I liked the sound of. The D’Addario sounded bright, glitzy, and artificial; the GHS was my choice for a darker sound among flat strings. However, it was difficult to judge whether they were good or bad because they did not work well with the bass I had put on at the time. I had the impression that it was difficult to find the perfect sound I was looking for among flat strings. While researching double bass strings, virtuoso Yuri Goloubev recommended a manufacturer called Tomastic, and I became interested in bass guitar strings from the same manufacturer. I bought them at Soundhouse for 9,280 yen(62 dollars). They are more expensive than other flat strings, but I have heard from numerous users that they have been used for over 10 years.
I put them on my Ibanez GSR180. The brightness of the round strings is gone, but that does not mean that the highs are not coming out, and the pitch sounds clear without being hidden when playing chords. On the other hand, unlike the tone of the round strings, the flat strings have a unique tight and dark sound. I got the impression that it was a combination of the best of both round and flat strings. Compared to the D’Addario and GHS that I have used in the past, the strings have the lowest tension and are easy to play. Also, I thought it was a well-balanced string that gives a mid-range that gives the contour of the sound which is different from GHS which is completely dedicated to dark tone. I am curious to hear how the sound will change after the break out.
9/21 Postscript: I noticed that the surface is stickier than other strings. It may be hard to play fast phrases.