I've been setting up my basses for years and never had this happen before. I put on a set of TI flats and noticed a doubling, almost chorus sound when I play an E - any E on the bass. There is the same thing on every A also, but not as loud. I did a setup, set the relief to the amount I use on all my basses (I eyeball it) and check to see if there was any buzzing. No buzzing, just the weird sound. I took the strings off and put the TI Superalloy that had been on there before, and it does the same thing, but not as loud. I hadn't noticed it with the SA, but the flats seemed to really bring it out. You can hear it acoustically too, so it isn't the electronics. Any idea what could cause that? edit: I should say that it is a '87 Gibson IV (not Thunderbird IV) with a set neck.