Currently, the only Ernie Ball flats I'm using are the Group IV's that the factory put on my Schecter Stiletto Studio 4-FL fretless bass. They're pretty well broken in, and on that bass? I like them. A very "modern", mid-range-y tone that the active electronics can really work with. Plus, they're flexible enough to work well on a fretless - but stiff enough for me to play with a pick. I'm used to playing really stiff flats that way; really flexible strings throw me off... Yeah, I know you're not "supposed" to play a fretless bass with a pick, but it's a lined fretless, and I usually play it like it has frets, anyway So...
As for the Slinky Cobalt Flats? I thought I had a couple sets of them (40-95 and 45-100), but for several years I didn't have a bass I wanted to try them on. The chance came when the old, salvaged set of D'Addario Chromes I had on a G&L ASAT Bass finally gave up the ghost. Unfortunately, turned out that my Slinky Cobalts - weren't Slinky Cobalts. They were very cleverly packaged counterfeits. So, into the dumpster they went, and the ASAT bass wears Fender 9050L's... And, TBH? The Fenders are just about as bright a flat as I want, anyway. Plus, every bass I own currently wears strings I like, and I'm too lazy - and not curious enough - to pull perfectly good flats off a bass, just to see if I might like some other strings. Which, from everyone's description of the Slinky Cobalts, I probably won't, anyway...