For years I steered way clear of Cobalt flats for that very reason!I had no problem with RS66. But the cobalt flats were like trying to play a cheese grater.
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I had a very bad experience with Cobalt rounds, which have absolutely positively the most aggressive surface roughness I've ever encountered on any bass string. No joke, 200 grit sandpaper would feel smoother. I had the rounds on one of my basses for less than an hour and they left a pile of brass shavings at the nut just from tuning. Slides were suicidal on the fingers and visible damage was being done on the frets in very short order so I ripped them off. Because of all that I swore I'd never try Cobalts again.
Then the Cobalt flats started getting rave reviews here, so I crossed my fingers and tried a set on my VP. Loved them! Gorgeous sound and silky smooth!
PillO - stupid question but I gotta ask since I experienced exactly what you did when I tried their rounds - are you sure you tried the flats? I have two sets of flats and both are dead smooth, as a flatwound should be. Again no joke, I still keep the Cobalt rounds around and now use them as nut files for fine tuning the grooves in metal, plastic and bone nuts. They do a good job for that.

