La Bella claims that their double ball strings are not for use on Spirit instruments. I emailed them to ask why. They replied that they really didn't know for certain whether or not the strings would work on a Spirit, and that I should contact Headless USA for a definitive answer. I wrote to Headless USA, and they responded that they hadn't had any complaints or returns from customers attempting to use La Bella strings on a Spirit. So I still don't have a conclusive answer. Since La Bella seems to be the only game in town for double ball flatwound strings, and since these strings are pretty expensive ($50 and up per set), I'd like to ask the hive mind at TalkBass if you've had experience with La Bella flatwounds (or any La Bella strings) on a Spirit bass, and if so, how it worked out for you. Thanks in advance!