They have a pleasant bite to them at first, but they aren't "zingy" like rounds and don't have the roundwound clank. It takes some people longer than others, but mine went through three stages:
1) Fresh out of the box - Still has some bite to it, but sounds like a mellowed set of rounds. You will probably go "I thought they would be thicker and heavier than this...maybe I made a bad choice." Do not fret, just enjoy the middle zinginess for now and know it will change soon.
2) Tonal Purgatory - Newness wears off by now and they are just starting to get settled/broken-in. They will sound like a dull set of rounds and will start to sound like they are lacking bassiness and response. They are mellow sounding, but aren't quite thumping yet. This is the hardest part for me to get past...this stage could easily make you restring, stop playing, or even sell the bass. Just hold out a little longer and you'll be glad you did! To speed things up try wiping sweat onto the strings, eating greasy food and wiping it up and down strings, adding a foam mute to the bridge, or leaving the bass out in the open without wiping it down after playing. The more you play it, the faster you will exit this phase
3) Thumptastic Euphoria - You've made it! Fingers or picked, you are getting those classic sounds you heard on the records (with the right amp, of course lol) from back in the day. Tone is thumpy with a hint of bite to let you know where you are on the fretboard. You sit well in the mix and can feel like a grown up since you broke in a pair of flats and didn't go crazy!