+1 to this. Not tried them myself but I've heard someone playing them and it did sound very string bass-like.
All the flats I've tried (la Bella, TI and chrome's) reduce finger noise and are pretty deep and full. Some are maybe a bit brighter than others but there's a 'family' sound to them all - which, to me, sounds like deeper, richer flats. Tape wounds sounded even more so - more like an upright.
But that could have also been the player, the guitar, the amp and eq and the general vibe (light jazz). How much part the strings actually played i don't know - but it was clearly a sound the guy was aiming for and he'd chosen nylon tapewounds to help him get it