I'd been using medium gauge nickel-wounds for ages, general D'Addario XLs - reasonable price, consistent, I've been happy. My most recent bass purchase was all dialed in on the intonation. Then I changed to ProSteels which were .005 thicker. I carefully dusted the nut slots open just a little so they would site in the bottom of the slots correctly. Intonation wasn't off much at the 12th and beyond but the lower register was 20-30hz/cents sharp. Thinking it was a string break issue I took the nut slots down more, and more, and more. I'm a former toolmaker so I am able to accertain I am down to about .015 over the height of the first fret now. Brought it in slightly but still not right. Out of disgust I changed back to some light gauge and it's dead-one (with great lower action). Maybe one of our luthier types could chime in - would frets be spaced slightly different for someone who uses really heavy stiff gauges? Anyone experienced this?