Normally, D'addario makes some of the best bass strings out there. They are cheaper than other brands and have a great tone to them, though they do not stay bright for as long as others do. However, in the case of the flats, you don't necessarily have that problem. These strings have, in my opinion, beaten out any others that I've used at some point (Fender, Rotosound). They sound really fat and clear. They work awesome with a P-bass if you're looking for the Motown vibe. Also, their sound is really remarkable when used in a J-bass context, fretted or fretless. Their medium range and tone works perfectly with the nasal nature of the fretless bass and, during the first two weeks or so of regular playing, you get an extra punchiness that helps you cut through the mix live thanks to the seal in the package.
These strings are an obvious choice for anyone looking for a good old sound.