It might have something to do with the setup you mentioned in your other thread. New strings are going to have a much brighter "zing" to them. Also if your bass was difficult to play before you may have compensated by digging in harder than necessary. A lower action will result in a bit of fret buzz or choking notes if played too hard. Try relaxing your hands and playing with a lighter touch and see if that changes anything. Another thing could be that with the setup and possibly different guage of strings your neck has settled too straight which will cause buzzing in the lower registers. Check your neck relief by pressing the first fret on the E or A string with your left hand and use your right hand index finger to press the last fret, then hold the bass to the light. The distance between the bottom of the string and the fret while doing this should be a around millimeter(or credit card thickness) at the 7th - 9th fret. If you don't see any gap, then the neck might be too straight. Try playing with a lighter touch first though and seeing if that makes a difference.