If all the beers taste oxidized, I suspect it's something else because a leaky pickup tube in the bottling bucket would only affect the last few bottles filled. When that tube is completely covered in beer, there's no way for oxygen to get in. Although I also tip the bucket at the end, so removing it won't hurt.
Do you use screw-type hose clamps on your tubing? I've found those clamps can crimp one side and actually make it easier for air to get in at that spot.