Confessions of a Cobal Flats lover...
Adored Cobalt flats on my VP so I put a set on my 2015 Bird. Wasn't as thrilled with the sound. Lowered the pups, raised the action, lifted the treble side of the pups, tweaked the amp settings, tried every possible option with the onboard controls. Just couldn't get the bass to sound right.
Pups up / action low = too boomy, chorus effect on some notes, phase cancellation on other notes, numerous dead spots, very uneven volume across the strings, sustain largely choked out.
Pups down / action high = too quiet, G and D strings completely lost in even a modest mix, all strings lost in a heavy mix even at full onboard volume.
Treble side of pups raised / action slightly lowered = about as good as it was gonna get, G and D strings increased volume, chorus effect minimal, phasing minimal, sustain ok, bass still completely lost in a heavy mix.
Finally had to say the heck with it and remove the Cobalts. Put on a set of Cocco nickel rounds and all appears to be going in the right direction. Sound has improved drastically. Much less mid focused, much better balance in volume across the strings, no obvious dead spots, no phasing or chorus effect. These strings appear to be a higher tension so I'll let them sit at pitch overnight before I make any neck adjustments. I suspect I'll be able to drop the action back down to where I like it (very low). I'm hoping I can raise the pups a bit as well to bump up the volume a bit.
Conclusion - as I eventually suspected, the magnets in these 2015 pups are monsters, Cobalt strings are exceptionally attracted to the magnets. There's nothing wrong with Cobalt strings or these pups, I just don't think they play well together. Are Cocco nickel rounds the perfect string for this bass? Probably not, I just happened to have an unused set laying around. I still have set up work to do as I dial in the new strings, but my string search for the 2015 will probably continue.