Alright, I'm currently in a battle with flats and I figure I should ask some people with a little more experience in the area than me.
I started playing in '08. The first bass I bought was, of course, strung up with rounds. An '08 Dean Playmate. Now, having just started, I found rounds to be a bit harsh on my fingers. After a couple months, I got my callouses going and they didn't bother me as much, but it still bothered me a little. Read up a little on flats, and got a set of Chromes. I enjoyed the smooth feel, but the tension compared to the rounds I was used to seemed a bit high. Finally, the tone. It was warm, full, but it seemed to be lacking punch, and was really zingy to start out. After a few weeks(I was a light player back then), it mellowed the zing out, but there was still a lack of some type of punch. This could have been caused by the pickup config. See, the bass had a single soapbar pickup, placed slightly closer to the neck than the average P pickup placement.
Later, I acquired a few other basses: An active double humbucker, a passive PJ, and a T-40. All of them were strung with rounds, mainly because I was exploring my options. I've gone through DR Fat Beams, Dunlop Nickels, and Rotosound Swing 66s. I liked the loose feel of all of them, but disliked the roughness.
So, my T-40 is looking for a new set, and I'm thinking about giving flats another shot. With the experiences I've described so far, can anyone point me towards a flatwound set that has a looser feel, like the roundwounds I've had experience with?