Daddario ECB84

I've putted them on my PRS Kestrel bass with single coil pickups.

Feeling is great, so different in compare to regular roundwound strings, They are smooth, movement of fingers is silent. Very interesting experience.

About the sound, they are warm and in compare to roundwounds, even dark. They have a great fundamental tone and power. For me it is OK, but I suspect that not every one will like it.

Slap and pop technique sounds good, but with flats you will lost this "zing" and percussive attack. Slap on flats is more like background playing.

Another thing is that flat strings keeps their sound for very long time.

IMO every bass player should try flats to see where is a difference.

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.