Hello,
I have a OLP Stingray and quite a problem. When I slap, all the strings sound fine except the A string which sounds quite weak and thin but I don't have that problem while playing regular fingerstyle. Also the problem is not in the strings because it's the 4th patch of strings I use in this bass.
Sorry if is this the wrong forum and I'll leave here a video to help you figure out what is the problem.
http://www.youtube.com/watch?v=a7A0O89d6A8